Make the cookies: Preheat oven to 325 degrees. Whisk together flour, cocoa, and salt in a bowl. In another bowl, cream butter and confectioners' sugar until pale and fluffy. Beat in egg and vanilla. Gradually add flour mixture, beating to combine. Shape dough into 2 disks, wrap each in plastic, and freeze until firm, about 15 minutes.
Working with 1 disk at a time, roll out dough on parchment paper to 1/8 inch thick. Freeze until very firm, about 15 minutes. Using a 1 1/2-inch biscuit cutter, cut out rounds. (Return dough to freezer if it softens.) Place rounds 1 inch apart on parchment-lined baking sheets. Freeze until firm, about 15 minutes. Brush off excess flour.
Bake cookies, rotating sheets once, until edges are firm, 12 to 14 minutes. Transfer cookies, on parchment, to a wire rack, and let cool completely.
Meanwhile, make the filling: Cream butter and shortening in the bowl of a mixer until well combined, about 2 minutes. With mixer on low speed, gradually add the confectioners' sugar, and beat until light and fluffy, about 2 minutes. Add the vanilla, and beat to combine. Set aside at room temperature until ready to use.
Place cream filling in a pastry bag fitted with a coupler, and pipe about 1 teaspoon filling onto the flat side of half the cookies. Place remaining cookies on top, and gently press on each to squeeze filling to edges. Filled cookies can be stored in airtight containers at room temperature up to 2 days. Just before serving, use a small paintbrush to apply a small amount of gold leaf to cookie tops.