Preheat your oven to 375°F (190°C). This ensures that your cake bakes evenly.

Grease two 8-inch round cake pans. Pour ¼ cup (60g) of melted butter into each pan, then sprinkle ¼ cup (50g) of sugar evenly in each pan. This creates a lovely caramelized layer at the bottom of your cake.

In a mixing bowl, combine the blueberries with the zest of one lemon. Spread the blueberry mixture evenly in both cake pans, making sure to distribute them well for even flavor.

Prepare the yellow cake mix according to the package instructions. Stir in the zest of the second lemon for an extra burst of flavor. Pour the batter evenly into the prepared cake pans.

Bake in the preheated oven for about 30 minutes, or until a toothpick inserted into the center comes out clean. Keep an eye on the cakes; ovens can vary.

Allow the cakes to cool in the pans for 10 minutes before carefully inverting them onto a platter. This helps them hold their shape.