Preheat and Prepare: Start by preheating your oven to 180 degrees Celsius (or 160 degrees for fan ovens). Grease a small baking sheet (approximately 28.5x23 cm) thoroughly to prevent sticking.
Melt Chocolate and Butter: Chop the dark chocolate into small pieces and place it in a bowl with the butter. Melt them together using a water bath or in the microwave, stirring frequently. Allow the mixture to cool for a few minutes.
Add Eggs and Sugar: Once the chocolate-butter mixture has cooled slightly, add the sugar and mix well. Then add the eggs one at a time, stirring until fully combined.
Combine Dry Ingredients: In a separate bowl, sift together the flour, cocoa powder, and salt. Gradually incorporate the dry ingredients into the chocolate mixture, folding with a spatula to avoid overmixing.
Fold in Chocolate Chunks: Roughly chop the whole milk and white chocolates, then fold about two-thirds of them into the batter.
Bake: Pour the brownie batter into the greased baking tray, smooth the top, and scatter the remaining chocolate chunks over the surface. Bake in the preheated oven for around 20 minutes, or until a toothpick inserted comes out with a few moist crumbs.
Cool and Serve: Remove the brownies from the oven and let them cool in the pan before cutting them into squares. Serve warm or enjoy them with a scoop of vanilla ice cream!