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1
Preheat oven to 325u00b0F and line a 13 x 9-inch baking dish with 1 sheet of parchment paper, allowing 2 inches to hang over each end.
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2
Place butter, sugar, cocoa powder, and salt in a large bowl and stir the mixture until combined. The texture will appear granular at this point. Stir in Bailey's and vanilla; add eggs and egg white one at a time, stirring well after each one. When the batter is thick and shiny, add the flour and stir until completely incorporated, then beat briskly for 40 more strokes. Spread batter evenly in lined pan.
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3
Bake for 18 to 20 minutes, until a toothpick inserted in the center comes out slightly moist with batter. Cool completely on a wire rack.
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4
For the topping, place mascarpone, Bailey's, and powdered sugar in a food processor and process until smooth. Spread evenly over brownie layer with an offset spatula. Cover and chill in the refrigerator at least 2 hours.
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5
When ready to serve, lift up the ends of the parchment and transfer the brownies to a cutting board, then cut into squares using a sharp knife and dust with cocoa. Store leftovers in an airtight container in the refrigerator up to 3 days.
