Preheat your oven to 180°C (350°F) and prepare an 8x8-inch baking tin by greasing it thoroughly and lining it with parchment paper.

In a medium mixing bowl, whisk together the plain flour, cocoa powder, baking powder, and salt until evenly blended.

In a separate large bowl, beat the melted unsalted butter with both white and brown sugars until the mixture is creamy and well-combined, about 2-3 minutes.

Gradually add the dry ingredients to your butter mixture, alternating with the whole milk, mixing gently until just combined.

Pour the prepared batter into the lined baking tin and bake for approximately 12 minutes or until a toothpick inserted comes out clean.

Once baked, allow the cake to cool in the tin for about 10 minutes before transferring it to a wire rack to cool completely.

Once cool, slice the cake in half horizontally and spread a generous layer of strawberry buttercream before topping with the other half.

To make cutting easier, place the layered cake in the freezer for about 30 minutes.

While the cake is freezing, make the ganache by heating the thickened cream until it simmers and then pouring it over the chopped dark chocolate.

Cut the solidified cake into bite-sized squares or wedges and dip each piece into the warm ganache.

Place them on a wire rack and top each petit four with a slice of fresh strawberry.