Step 1: Prepare the Lemon Cake Layers

1. Preheat Oven and Prepare Pans: Preheat your oven to 350°F (175°C). Grease and flour three 8-inch round cake pans. You can also line the bottoms with parchment paper for easier removal. This prevents sticking and ensures even baking.
2. Combine Dry Ingredients: In a medium bowl, whisk together the all-purpose flour, baking powder, baking soda, and salt. Whisking ensures even distribution of the leavening agents and salt throughout the flour. Set aside.
3. Cream Butter and Sugar: In a large bowl or the bowl of a stand mixer fitted with the paddle attachment, cream together the softened butter and granulated sugar until light and fluffy. This process incorporates air into the mixture, which is essential for a light and tender cake. Cream for 3-5 minutes until the mixture is pale and noticeably fluffy.
4. Incorporate Eggs: Beat in the eggs one at a time, mixing well after each addition. Ensure each egg is fully incorporated before adding the next. This helps create a smooth and emulsified batter.
5. Combine Wet Ingredients: In a separate liquid measuring cup, whisk together the buttermilk, vegetable oil, lemon zest, and lemon juice. This pre-mixing ensures even distribution of the liquid ingredients into the batter.
6. Alternate Wet and Dry Ingredients: Gradually add the dry ingredients to the wet ingredients in three additions, alternating with the wet ingredients in two additions, beginning and ending with the dry ingredients. Beat on low speed until just combined after each addition. Be careful not to overmix, as this can lead to a tough cake. Mix until just incorporated, leaving a few streaks of flour is okay, they will disappear with the final mixing.
7. Stir in Rosemary: Gently fold in the finely chopped fresh rosemary until evenly distributed throughout the batter. Folding ensures the rosemary is incorporated without overmixing the batter.
8. Divide Batter and Bake: Divide the batter evenly among the prepared cake pans. Use a kitchen scale for precise division to ensure even layers. Bake for 25-30 minutes, or until a toothpick inserted into the center comes out clean. The baking time may vary depending on your oven. Start checking for doneness around 25 minutes.
9. Cool Cakes: Let the cakes cool in the pans for 10-15 minutes before inverting them onto a wire rack to cool completely. Cooling in the pans prevents the cakes from breaking apart while still warm. Ensure the cakes are completely cool before frosting to prevent the frosting from melting.

Step 2: Prepare the Lemon-Rosemary Syrup

1. Combine Syrup Ingredients: In a small saucepan, combine the granulated sugar, water, lemon juice, and rosemary sprigs.
2. Simmer and Infuse: Bring the mixture to a simmer over medium heat, stirring until the sugar is dissolved. Reduce the heat to low and simmer for 5-7 minutes to allow the rosemary to infuse the syrup. Simmering concentrates the flavors and creates a syrupy consistency.
3. Strain and Cool: Remove the saucepan from the heat and strain the syrup through a fine-mesh sieve to remove the rosemary sprigs. Let the syrup cool completely before using. Cooling the syrup prevents it from melting the frosting and ensures it soaks into the cake layers properly.

Step 3: Prepare the Lemon Buttercream Frosting

1. Beat Butter: In a large bowl or the bowl of a stand mixer fitted with the paddle attachment, beat the softened butter until smooth and creamy. This creates a smooth base for the buttercream. Beat for 2-3 minutes until light and fluffy.
2. Add Powdered Sugar: Gradually add the sifted powdered sugar, one cup at a time, beating on low speed until combined. Increase the speed to medium and beat until light and fluffy, scraping down the sides of the bowl as needed. Adding powdered sugar gradually prevents it from creating a cloud and ensures it mixes in smoothly.
3. Incorporate Wet Ingredients: Gradually add the heavy cream (or milk), lemon juice, vanilla extract, lemon zest, and salt. Beat on medium speed until the frosting is smooth, light, and fluffy. Adjust the amount of liquid to achieve your desired consistency. If the frosting is too thick, add a little more liquid; if it’s too thin, add a little more powdered sugar.
4. Whip for Fluffiness: Continue to beat the frosting for another 2-3 minutes until it is very light and fluffy. This incorporates more air into the frosting, making it even lighter and smoother.

Step 4: Assemble the Cake

1. Level Cake Layers (Optional): If your cake layers are slightly domed, use a serrated knife to level the tops for a flat and even surface. This ensures the cake layers stack evenly and the frosting is applied smoothly.
2. Syrup Soak: Place one cake layer on a serving plate or cake stand. Generously brush the top of the cake layer with the lemon-rosemary syrup. This step adds moisture and enhances the lemon-rosemary flavor throughout the cake.
3. Frost First Layer: Spread a layer of lemon buttercream frosting evenly over the syrup-soaked cake layer.
4. Repeat Layers: Top with the second cake layer, soak with syrup, and frost with buttercream. Repeat with the final cake layer, soaking it with syrup as well.
5. Frost the Cake: Frost the top and sides of the cake with the remaining lemon buttercream frosting. You can create a smooth finish or decorative swirls, depending on your preference.
6. Chill (Optional): For easier slicing and serving, chill the frosted cake in the refrigerator for at least 30 minutes before serving. Chilling firms up the frosting and makes the cake more stable.
7. Garnish (Optional): Garnish the cake with fresh rosemary sprigs, lemon slices, or candied lemon zest for an elegant presentation.