Preheat the oven to 350°F (175°C) and grease a 9-inch round cake pan.

In a mixing bowl, whisk together the flour, sugar, baking powder, baking soda, and salt.

In another bowl, mix the buttermilk, egg, melted butter, lemon juice, crushed lavender, and lemon zest until well combined.

Pour the wet ingredients into the dry ingredients and mix until just combined.

Pour the batter into the prepared cake pan and smooth the top.

Bake for 25 minutes or until a toothpick inserted in the center comes out clean.

Allow the cake to cool in the pan for 10 minutes, then transfer to a wire rack to cool completely.

In a mixing bowl, beat the softened butter until creamy.

Gradually add the powdered sugar, mixing on low speed until combined.

Add the lemon juice and vanilla extract, and beat on high speed until fluffy.

Once the cake is completely cool, spread the frosting evenly over the top and sides of the cake.

Garnish with dried culinary lavender on top.

Slice and serve the cake chilled or at room temperature.