Preheat oven to 350°F (175°C). Grease and flour two 8-inch round cake pans, then line the bottoms with parchment paper.

In a medium bowl, whisk together the flour, baking powder, and salt. Set aside.

In a large bowl with an electric mixer, cream together the softened butter and granulated sugar on medium speed until light and fluffy, about 3-5 minutes.

Add eggs one at a time, beating well after each addition. Scrape down the sides of the bowl as needed.

Stir in the lemon zest and crushed dried culinary lavender until just combined.

In a separate small bowl, whisk together the buttermilk and vanilla extract.

Gradually add the dry ingredients to the wet ingredients in three additions, alternating with the buttermilk mixture in two additions, beginning and ending with the dry ingredients. Mix on low speed until just combined after each addition; do not overmix.

Divide the batter evenly between the two prepared cake pans. Bake for 28-32 minutes, or until a wooden skewer inserted into the center comes out clean.

Let the cakes cool in the pans for 10-15 minutes, then invert them onto a wire rack to cool completely.

While cakes are cooling, combine sugar, water, 2 tbsp lemon juice, and 1 tsp dried culinary lavender in a small saucepan.

Heat over medium heat, stirring until sugar dissolves. Bring to a gentle simmer, then remove from heat. Let steep for 10-15 minutes, then strain the syrup through a fine-mesh sieve to remove lavender buds. Discard lavender.

Let the simple syrup cool completely.

In a large bowl with an electric mixer, beat the softened butter on medium speed until creamy, about 2 minutes.

Gradually add the sifted powdered sugar, 1 cup at a time, beating on low speed until combined, then increase speed to medium-high and beat until fluffy.

Add 3 tbsp fresh lemon juice, lemon zest, and a pinch of salt. Beat until light and fluffy, adding more lemon juice (up to 4 tbsp total) if needed to reach a desired consistency.

If desired, add a few drops of purple food coloring and mix until evenly colored.

Once the cakes are completely cool, use a serrated knife to level the tops of the cakes if necessary. Place one cake layer on your serving plate or cake stand.

Brush generously with half of the cooled lemon lavender simple syrup.

Spread about 1 cup of lemon buttercream frosting evenly over the first layer.

Carefully place the second cake layer on top.

Brush generously with the remaining simple syrup.

Frost the top and sides of the cake with the remaining buttercream.

Garnish with fresh lemon slices and lavender sprigs, if desired.