First, preheat the oven and prepare your cake pans by greasing and lining them. In a large bowl, beat the butter and sugar together until light and fluffy. Add eggs and vanilla, mixing until fully combined. Gradually add the dry ingredients and milk, alternating between them, until the batter becomes smooth and creamy. Gently fold in the fresh blueberries to keep them evenly distributed.
Divide the batter evenly between the prepared pans. Bake until the cakes turn golden and a toothpick inserted in the center comes out clean. Once baked, allow the cakes to cool in the pans for a short time before transferring them to a wire rack. Let them cool completely before frosting to prevent melting.
To prepare the syrup, simmer water, sugar, and dried lavender together. Let it steep so the floral flavor infuses the mixture, then strain and cool it. For the buttercream, beat the butter until creamy, gradually add powdered sugar and milk, and mix in mashed blueberries until smooth and lightly colored.
Brush each cooled cake layer with lavender syrup to enhance moisture and flavor. Spread a generous layer of blueberry buttercream on the first layer, then place the second layer on top. Frost the entire cake evenly and decorate as desired. Chill slightly before serving for clean slices.