Cream butter and sugar together until light and fluffy. Beat in egg, then vanilla.

In a separate bowl, whisk together flour, cocoa and salt. Add to butter mixture and beat to combine. Transfer dough to refrigerator to chill for 1 hour (or up to 2 days before baking).

While dough is chilling, prepare filling. Heat whipping cream to a simmer in the microwave (about 1 1/2 minutes on high power). Pour over chocolate and whisk until smooth. Cool in refrigerator until spreadable.

Preheat oven to 350ºF. Working with one half of the dough at a time, form a disc then roll it out to 1/4 inch thickness between two sheets of parchment paper. Use a 2-inch cookie cutter to cut out shapes. Transfer to baking sheets. Decorate with colored sugar or sprinkles, if desired.

Bake one sheet at a time for 9 to 11 minutes, until cookies are firm. Cool 2 minutes on sheet before transferring to wire racks to cool completely.

Once cookies have cooled, spread half of them with about 2 tsp of ganache filling and top with the remaining cookies.