Preheat Oven:

Set oven to 350°F (175°C). Line a 9x9-inch baking pan with parchment or lightly grease it.

Melt Butter & Chocolate:

In a saucepan over low heat (or in the microwave), melt butter with chopped chocolate. Stir until smooth. Let cool slightly.

Mix Wet Ingredients:

In a large bowl, whisk together the eggs, both sugars, and vanilla. Slowly stir in the melted chocolate-butter mixture.

Add Dry Ingredients:

In a separate bowl, whisk together flour, cocoa powder, salt, and baking powder. Gently fold into the wet mixture until just combined.

Optional Add-Ins:

Fold in chocolate chips or nuts, if using.

Bake:

Pour the batter into your prepared pan and smooth the top. Bake for 30–35 minutes, or until a toothpick inserted near the center comes out with moist crumbs (not wet batter).

Cool & Slice:

Allow to cool in the pan for at least 20 minutes before slicing into squares.