Preheat your oven to 350°F (175°C) and grease a 9x9-inch baking pan with butter or dairy-free butter.

In a heatproof bowl over a pot of simmering water, melt the dark chocolate and butter or coconut oil, stirring until smooth.

Whisk the eggs and granulated sugar together until glossy and pale, about 2-3 minutes.

Gently fold the melted chocolate mixture into the egg and sugar blend until just combined.

Sift the cocoa powder and all-purpose flour over the batter, folding gently until just combined.

Transfer the batter into the prepared baking pan and smooth the top. Add any optional add-ins.

Bake for 25-30 minutes, until the edges are set and center is slightly soft.

Cool in the pan for about 30 minutes, then slice into squares and serve.