Part 1: Baking the Raspberry Cake Layers

1. Preheat and Prepare: Preheat your oven to 350°F (175°C). Grease and flour two 8-inch or 9-inch round cake pans. Alternatively, line the bottoms with parchment paper, then grease and flour the sides. This ensures easy release.
2. Combine Dry Ingredients: In a large mixing bowl, whisk together the 2 ½ cups of all-purpose flour, granulated sugar, baking powder, baking soda, and salt. Whisking helps to aerate the flour and evenly distribute the leavening agents, leading to a lighter cake.
3. Combine Wet Ingredients: In a separate medium bowl, whisk together the melted butter, large eggs (one at a time, ensuring each is incorporated), buttermilk, vegetable oil, and vanilla extract until well combined and smooth. Using room temperature eggs and buttermilk helps them emulsify better with the fats, resulting in a more tender crumb.
4. Combine Wet and Dry: Gradually pour the wet ingredients into the dry ingredients. Mix on low speed with an electric mixer or by hand with a whisk until just combined. Be careful not to overmix at this stage; a few small lumps are okay. Overmixing develops gluten, which can make the cake tough.
5. Prepare Raspberries: In a small bowl, gently toss the fresh raspberries with the remaining 1 tablespoon of flour. This light coating helps prevent the raspberries from sinking to the bottom of the cake batter during baking.
6. Fold in Raspberries: Gently fold the flour-dusted raspberries into the cake batter using a rubber spatula. Distribute them as evenly as possible without overmixing.
7. Divide and Bake: Evenly divide the batter between the prepared cake pans. Smooth the tops with a spatula.
8. Bake: Bake for 30-35 minutes for 8-inch pans, or 25-30 minutes for 9-inch pans, or until a wooden skewer or toothpick inserted into the center of the cakes comes out clean or with a few moist crumbs attached. The tops should be golden brown and spring back when lightly touched.
9. Cool Cakes: Once baked, let the cakes cool in their pans on a wire rack for 10-15 minutes. Then, carefully invert the cakes onto the wire rack to cool completely. Ensure they are fully cooled before attempting to frost, otherwise, the buttercream will melt. This can take 1-2 hours.

Part 2: Making the Luscious Lemon Buttercream

1. Beat Butter: In a large mixing bowl, using an electric hand mixer or a stand mixer fitted with the paddle attachment, beat the softened unsalted butter on medium-high speed for 3-5 minutes until it’s very pale, light, and fluffy. This step is crucial for a light buttercream.
2. Add Powdered Sugar: Gradually add the sifted powdered sugar, one cup at a time, beating well on low speed after each addition. Once all sugar is incorporated, increase the speed to medium-high and beat for another 2-3 minutes until smooth. Start with 4 cups and add more if a stiffer consistency is desired.
3. Incorporate Lemon: Add the fresh lemon zest, 3 tablespoons of fresh lemon juice, vanilla extract, and the pinch of salt. Beat on low speed until combined, then increase to medium-high and beat for another 2 minutes until the buttercream is light, airy, and voluminous.
4. Adjust Consistency (if needed): Taste the buttercream. If you desire a stronger lemon flavor, add the remaining tablespoon of lemon juice. If the buttercream is too thick, add heavy cream or milk, one teaspoon at a time, until you reach your desired consistency. If it’s too thin, add a bit more sifted powdered sugar.
5. Final Beat: Give the buttercream one last vigorous beat for about a minute to ensure it’s perfectly smooth and fluffy.

Part 3: Assembling Your Raspberry Cake with Lemon Buttercream

1. Level Cakes (Optional but Recommended): Once the cake layers are completely cool, use a long serrated knife to level the tops if they have domed during baking. This creates a flat, stable surface for stacking and frosting.
2. First Layer: Place one cake layer, cut-side up (if leveled), onto a cake stand or serving plate.
3. Frost First Layer: Scoop about 1 to 1 ½ cups of the lemon buttercream onto the center of the first cake layer. Using an offset spatula, spread the buttercream evenly over the top, right to the edges. You can create a small “dam” of frosting around the edge if you plan to add a raspberry filling, but for this recipe, a generous layer of buttercream is perfect.
4. Add Second Layer: Carefully place the second cake layer on top of the frosted first layer. You can place it cut-side down for a neater top surface or cut-side up if you leveled both layers. Gently press down to secure it.
5. Crumb Coat (Optional but Recommended): Apply a thin, even layer of buttercream over the top and sides of the entire cake. This is called a “crumb coat” and it traps any loose crumbs, ensuring your final layer of frosting is smooth and crumb-free. Refrigerate the crumb-coated cake for about 20-30 minutes to allow the frosting to set.
6. Final Frosting: Once the crumb coat is set, apply the remaining lemon buttercream generously over the top and sides of the cake. Use an offset spatula or a bench scraper for a smooth finish, or create decorative swirls with the back of a spoon or a spatula.
7. Garnish: Decorate your beautiful Raspberry Cake with Lemon Buttercream. Fresh raspberries artfully arranged on top, perhaps with a few thin lemon slices or zest curls, and a sprig of mint, look stunning.
8. Chill (Optional): For cleaner slices, you can chill the fully frosted cake in the refrigerator for at least 30 minutes before serving. This helps the buttercream firm up.