Preheat the oven to 350F.
In a small saucepan, melt the butter over medium-high heat. (If you want browned butter, keep it on the heat, swirling the pan occasionally, until the foam starts turning golden and the mixture smells nutty.) Pour into a medium bowl and stir in the brown sugar, egg and vanilla. Add the flour, baking soda and salt and stir just until combined. If you like, add a handful of chopped chocolate (or chips).
Line an 8×8-inch pan with parchment (or butter it) and put the cocoa in a shallow dish. Scoop up large spoonfuls of batter and dip the side of the spoon in the cocoa, then drop the batter into the pan—the idea is to get some cocoa interspersed throughout the batter. Draw the tip of a skewer or knife through to create a swirled effect and bake for 20-25 minutes, until golden and just set.
Makes 12-16 blondies.