Beat butter and powdered sugar until light and fluffy, about 2 minutes. Add vanilla and salt, beat to combine.

Sift cocoa powder and flour into the bowl, then mix on low just until the dough comes together in a smooth ball.

Shape the dough into a 2-inch-wide log, wrap tightly in plastic, and refrigerate at least 2 hours until firm.

Preheat oven to 325°F (160°C). Slice the log into 1/4-inch rounds, place on a parchment-lined sheet, and bake 12 to 15 minutes until edges are set. Cool on the sheet 5 minutes, then transfer to a wire rack.