Preheat the oven to 180°C (350°F) and grease a 9-inch square baking pan.
In a bowl, mix 200 grams of all-purpose flour, 50 grams of cocoa powder, 1 teaspoon of baking powder, and a pinch of salt. Set aside.
In another bowl, melt 200 grams of dark chocolate and 200 grams of unsalted butter. Mix well.
Add 300 grams of sugar to the chocolate-butter mixture and mix until well combined.
Add 3 large eggs, one at a time, mixing well after each addition.
Gradually add the dry ingredients to the wet ingredients, mixing until just combined.
Pour half of the brownie batter into the prepared pan and spread evenly.
Place 4-6 small, store-bought lava cakes (or homemade lava cakes) on top of the brownie batter, spacing them evenly.
Pour the remaining brownie batter over the lava cakes, covering them completely.
Bake for 35-40 minutes, or until a toothpick inserted in the center comes out with a few moist crumbs.
Allow the brownie and lava cake to cool in the pan for 10 minutes before cutting into squares and serving.