Preheat your oven to 350 degrees Fahrenheit (177 degrees Celsius). This temperature is crucial for achieving that perfect rise and golden color. Prepare three 8-inch round cake pans by lining them with parchment paper and greasing the sides for easy removal.
In a large mixing bowl, combine 3/4 cup of softened unsalted butter and 1 cup of granulated sugar. Using a hand mixer, cream them together for 2 to 3 minutes, or until the mixture is light and fluffy. This step is essential for incorporating air, leading to a soft cake.
Add the egg yolks, 1 tsp of vanilla extract, and 1 zest of lemon to the butter mixture. Mix again until it's creamy and smooth. Then, slowly pour in 1 cup of lukewarm milk, but don’t mix just yet. This allows the flavors to meld beautifully.
In a separate bowl, combine all your dry ingredients: 1 1/2 cups of all-purpose flour, 3/4 cup of almond flour, 4 tsp of baking powder, and 1/4 tsp of salt. Sift these dry ingredients into your wet mixture, and use a whisk or the hand mixer to combine. Be careful not to over-mix; just blend until the flour is incorporated.
In another bowl, whisk the egg whites on high speed for about 2 to 3 minutes, until they form medium peaks. This step will add volume and lightness to your cake. Gently fold the whipped egg whites into the cake batter using a spatula. Be gentle to maintain the airiness of the mixture.
Divide the batter evenly between the three prepared cake pans, using a kitchen scale if you want to be precise. Toss 1 cup of fresh blackberries in 1 tbsp of corn starch to coat them, then drop 6 to 8 berries into each pan, spacing them out for even distribution.
Bake in the preheated oven for about 25 minutes, or until a toothpick inserted into the center comes out clean. Keep an eye on them; you want a light golden color on top. Once done, remove the pans from the oven and cool them completely on a wire rack.
While the cake layers are cooling, make the French buttercream. In a stand mixer bowl, whisk together the 5 egg yolks on high speed for about 2 to 3 minutes until thick and pale. This is crucial for the structure of your buttercream.
Meanwhile, in a small saucepan, combine 1/3 cup of water and 1 cup of granulated sugar. Stir gently to combine, then attach a candy thermometer and cook over medium heat until it reaches 238 degrees Fahrenheit (114 degrees Celsius), about 7 to 8 minutes.
Once the syrup is ready, remove it from the heat. With the mixer on medium speed, slowly pour the hot syrup into the whisked egg yolks. Be sure to pour it away from the whisk attachment to avoid crystallized sugar bits forming. After all the syrup is added, increase the speed to high and whisk until the egg yolks double in size and the mixture cools down to lukewarm.
Now, begin adding the 2 cups of softened unsalted butter, one tablespoon at a time, mixing well after each addition. This process takes about 8 to 10 minutes. The butter needs to be incorporated slowly to prevent the cream from separating. As you add more butter, the cream will thicken beautifully.
Finally, add 1/4 tsp of lavender oil and a few drops of purple food coloring if desired. Once the cake layers are completely cool, spread a generous layer of buttercream between each layer. Smooth the remaining frosting over the top and sides of the cake. Garnish with extra blackberries, lavender sprigs, and edible greens if you like.
