Using an electric mixer, cream the sugar, butter and salt at high speed until well blended, about 3 minutes. Scrape down the bowl, add the egg and vanilla and beat for 1 minute.
Beat in the cocoa at medium speed until thoroughly blended. Add the flour and mix at low speed just until incorporated.
Preheat the oven to 350 degrees . Line 2 cookie sheets with parchment.
Scoop out 1 tbsp. of dough, roll into a ball and place on a prepared pan. Repeat with the remaining dough, spacing evenly on the pans.
Using the bottom of a drinking glass, flatten each ball into a 1/4-inch-thick round. Peel the dough off the glass; return to the pan. Dip a small paintbrush into the edible gold dust; flick onto the cookies.
Bake 1 pan at a time (refrigerate the other pan) until the cookies are set at the edges, 10 to 11 minutes. Let cool on the pan for 2 minutes; transfer to racks to cool completely.