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Preheat oven to 375F. Line a baking sheet with parchment paper.
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Use a mixer to cream the softened butter, peanut butter, and brown sugar until light and fluffy. Add the milk, vanilla, and egg and beat until combined.
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In a separate bowl, sift the flour, cocoa, salt and baking soda. Add to butter mixture low speed. Mix just until blended.
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Working with about a teaspoon of batter, roll into balls; place onto prepared sheet. Dip the tines of a fork into the sugar and press into the cookie to make a crisscross design. Continue for each cookie.
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Bake for 7 to 8 minutes, or until set and just beginning to brown. Cool on a wire rack.