WHISK flour, cocoa, baking powder and salt in a medium bowl.
BEAT butter and sugar in a large bowl, using an electric mixer, until fluffy. Beat in egg and vanilla. Using a wooden spoon, gradually stir in flour mixture just until dough comes together.
DIVIDE dough into 2 portions. Shape each into an 6-in round or square or triangular log.
WRAP each with plastic wrap. Refrigerate until firm, at least 1 hour or up to 1 week.
PREHEAT oven to 350F. Line 2 large baking sheets with parchment. Cut logs into 1/8-in. thick slices. Arrange 1-in apart on prepared sheets. Bake until cookies are no longer shiny, about 12 min. Let stand on baking sheet 2 min. Remove cookies to a rack to cool completely. Repeat with remaining dough. Spread bottoms of half of cookies with 1/2 tsp Speculoos butter. Sandwich with remaining cookies.
ADD 1/2 tsp gold dust to small bowl. Add 1/4 tsp vodka, stirring just until thin enough to paint with. Brush tops of sandwich cookies with gold paint. Cookies will keep well in a covered container at room temperature up to 2 days.
How to roll icebox cookie dough
Nutrition (per cookie)
Kitchen tip: Turn log every 30 min if forming into square or triangle to create sharp corners.
Prep Tip: Use the length of a ruler to press log into square or triangle shapes.
Tip: Overwrap log with foil and freeze up to 1 month.