Bring butter to room temperature and use a mixer to blend with light brown sugar and powdered sugar until creamy, about 3-4 minutes.

Sift together black cocoa powder, natural cocoa powder, and powdered sugar. Gradually add to the cream mixture and blend until combined.

Sift in all-purpose flour slowly while mixing on low speed until just combined, aiming for a thick, cohesive dough.

Transfer the dough to a lined baking pan and press evenly down to form a level layer.

Preheat oven to 325°F (163°C) and bake for 15-40 minutes, checking until cookies appear matte.

Cool the pan for 20-30 minutes, then cut cookies into squares using a serrated knife.