For the brownies:
10 tbsp. unsalted butter, at room temperature
1 cup sugar
¾ cup plus 2 tbsp. unsweetened cocoa powder
¼ tsp. salt
½ tsp. vanilla extract
2 large eggs, cold
½ cup all-purpose flour
For the frosting:
6 tbsp. unsalted butter, at room temperature
¾ cup creamy peanut butter
1 cup confectioners’ sugar
Pinch of salt
Additional sea salt, such as fleur de sel, for sprinkling
Preheat the oven to 325˚ F. Line an 8 x 8-inch baking pan with lightly greased parchment paper. Cut the butter into chunks and place in large heatproof bowl. Add the sugar, cocoa powder and salt to the bowl. Place the bowl over a pot of simmering water so that the steam from the water heats the contents of the bowl (double boiler method). Stir occasionally, heating until the butter is fully melted. Whisk until evenly combined. Set aside and let cool briefly until just warm to the touch (not hot).
Stir in vanilla and then the eggs one at a time, whisking just until smooth. Fold in the flour just until incorporated and no streaks remain. Spread the batter into the prepared pan and bake 20-25 minutes, until just set. Transfer to a wire rack and let cool completely before proceeding.
To make the frosting, combine the butter and peanut butter in the bowl of an electric mixer. Beat on medium-high speed until smooth, about 2 minutes. Blend in the confectioners’ sugar and salt on low speed, then increase to medium-high and whip until light, fluffy and completely smooth, about 3-4 minutes.
Spread the peanut butter frosting in an even layer over the cooled brownies. Sprinkle with additional sea salt. Slice into bars and serve.