In a mixing bowl, cream the butter and sugar together just until its combined.
With the mixer running on low, add the room temperature egg and vanilla. Mix until completely combined.
Add the flour, salt, baking powder, baking soda, and cocoa powder. Mix until just combined.
On a parchment lined baking tray, use a heaping tablespoon to scoop the dough into balls. Roll the balls to make them round then gently press your thumb into the center to flatten the cookie and create the center for the filling. Repeat with all of the dough.
Pour some of the Domino® Golden Sugar into a small bowl and gently roll each cookie ball completely in the sugar. Place back on the baking tray and freeze for 30 minutes, or until solid.
Preheat the oven to 375, bake the cookies for 8 minutes, then remove and place on a wire cooling rack. Carefully, use your thumb to re-indent the cookies after they have puffed up in the oven. Allow to cool completely before moving.