Preheat the oven to 350°F (175°C). Line an 8x8-inch (20x20 cm) square baking pan with parchment paper leaving an overhang for easy removal, or grease the pan lightly.

Melt chocolate and butter

Place the chopped chocolate and butter in a heatproof bowl set over a gently simmering pot of water (double boiler) or melt in 20-30 second bursts in the microwave, stirring between bursts until smooth. Remove from heat and let cool for 3–5 minutes.

Combine sugar and eggs

In a medium bowl, whisk the granulated sugar and eggs together until combined and slightly thickened. Stir in the vanilla extract.

Mix wet and dry

Pour the melted chocolate-butter mixture into the egg-sugar mixture and stir until well combined. Sift in the flour, cocoa powder, and salt directly into the bowl and fold with a spatula just until no streaks of flour remain. Do not overmix.

Add nuts (optional)

Fold in the chopped walnuts or pecans if using.

Bake

Spread the batter evenly into the prepared 8x8 pan, smoothing the top. Bake in the preheated oven for 22–28 minutes, until a toothpick inserted into the center comes out with a few moist crumbs (not wet batter) for fudgy brownies or drier crumbs for cakier brownies.

Cool and slice

Allow the brownies to cool completely in the pan on a wire rack (about 1 hour) to set. Use the parchment overhang to lift the brownie slab out of the pan and slice into 9 or 12 pieces with a sharp knife. For cleaner slices, chill 30 minutes before cutting and wipe the knife between cuts.

Vanilla Whipped Cream (optional topping)

Chill a mixing bowl and whisk or beaters in the refrigerator for 10–15 minutes if possible. Pour the heavy cream into the chilled bowl, add powdered sugar and vanilla extract. Whip with a hand mixer or whisk by hand until soft peaks form, 2–4 minutes. Do not overbeat. Serve a dollop atop each brownie slice.