Preheat your oven to 350°F (177°C). Grease and line two 8-inch round cake pans with parchment paper.
In a medium bowl, whisk together the flour, baking powder, crushed lavender, and salt.
In a large bowl, use an electric mixer to cream the softened butter and granulated sugar together until light and fluffy, about 3-5 minutes.
Beat in the whole eggs one at a time, followed by the egg yolks and vanilla extract.
Gradually add the dry ingredients and buttermilk, alternating between them, mixing on low speed until just combined.
Divide the batter evenly between the prepared pans. Bake for 25-30 minutes, or until a toothpick inserted in the center comes out clean.
After baking, let the cakes cool in the pans for 10 minutes before turning out onto a wire rack to cool completely.
While the cakes bake, combine the sugar, water, and lavender in a saucepan to make lavender syrup. Simmer until sugar dissolves, then steep and strain.
Beat the softened butter for the frosting until creamy. Gradually mix in the sifted powdered sugar, lavender syrup, and vanilla extract until smooth.
Notes
Opt for high-quality culinary lavender and ensure other ingredients are at room temperature for best results.