Preheat the 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 dry ingredients: flour, baking powder, crushed lavender, and salt.

Cream together the butter and sugar until light and fluffy, then add the eggs, egg yolks, and vanilla extract.

Gradually add the dry ingredients and buttermilk to the butter mixture, starting and ending with the dry ingredients.

Divide the batter into the prepared pans and bake for 25 to 30 minutes, or until a toothpick comes out clean.

Allow the cakes to cool in the pans for 10 minutes, then remove and cool completely on a wire rack.

Prepare the lavender simple syrup by simmering water and sugar, then add crushed lavender and steep.

Beat softened butter, add powdered sugar and lavender syrup to make the frosting.

Assemble the cake by layering, frosting between each layer and on the top and sides. Chill briefly to set.