Preheat oven to 160°C. Butter a 9-inch square baking pan and line with parchment, leaving a 2-inch overhang on both sides. Butter the parchment as well.

Melt butter and Callebaut Dark Callets™ together. Remove from heat and set them aside to cool.

In a large bowl, whisk together eggs, sugar, and vanilla extract to a glossy, smooth sabayon (creamy sauce).

Fold in the cooled chocolate mixture. Using a rubber spatula, stir the chocolate mixture with the sabayon until just combined.

Sieve flour and cocoa powder three times before folding them into the mixing bowl.

Fold in the remaining Callebaut Milk Callets™.

Transfer batter to baking pan, smoothing the top with a spatula.

Bake for about 30 minutes, until the top is shiny and crackly. A rule of thumb is your tester comes out clean.

Transfer the pan to a wire rack and let it cool for 20 minutes. Using parchment overhang, lift brownies from pan before cutting into squares.