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In your food processor, pulse flour, cocoa powder, powdered sugar, and salt until combined.
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Add butter and pulse mixture resembles coarse cornmeal, with butter pieces no larger than small peas. Add the egg and pulse just until the dough pulls together. Do not over process.
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Alternatively, make combine dry ingredients in a large bowl. Cut butter into flour mixture using a pastry cutter. Add egg and vanilla and mix until dough comes together.
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Evenly press dough into a 9-inch tart pan with removable bottom.
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Wrap entire pan in plastic wrap and freeze for 1 hour.
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Once frozen, preheat oven to 425 degrees.
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Remove plastic wrap and bake tart for 15-18 minutes or until it appears dry.
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Remove from oven and cool on a wire cooling rack.