Preheat your oven to 350°F (175°C) and prepare a 9x13-inch baking pan with parchment paper and a light grease.

In a large mixing bowl, whisk together the flour, baking powder, and salt until combined.

In a separate bowl, cream together the butter and sugar until light and fluffy. Add eggs one at a time and mix in the vanilla extract.

Gradually add the flour mixture to the wet ingredients, alternating with the milk, mixing gently until just blended.

Spread the batter in the prepared pan and bake for 18-22 minutes, or until a toothpick comes out clean.

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

Prepare the blueberry filling by cooking blueberries, sugar, and lemon juice until bubbling. Add a cornstarch slurry to thicken, then let cool.

Assemble the cakes by layering the sponge shapes with blueberry filling in between.

Chill the assembled cakes in the refrigerator for 20-30 minutes.

Make the lavender glaze by infusing the milk with culinary lavender, straining, and whisking with powdered sugar.

Glaze the chilled cakes and let them set for 10-15 minutes before serving.

Add final touches with fresh blueberries or dried lavender for decoration.