In a large mixing bowl, cream together softened butter and granulated sugar on medium speed until light and fluffy, about 3-4 minutes.

Beat in egg and vanilla extract, mixing until just combined.

In a separate bowl, whisk together flour, cocoa powder, and salt. Gradually add this mixture to the butter mixture. Mix on low speed until combined.

Transfer the dough to the refrigerator and chill for 1 hour (or up to 2 days if you want to prepare in advance).

Heat heavy whipping cream in the microwave on high for about 1 1/2 minutes until simmering.

Pour the hot cream over the chopped chocolate and whisk until smooth and glossy. Allow the ganache to cool in the refrigerator until it’s spreadable.

Preheat your oven to 350ºF (175ºC).

With half the chilled dough at a time, form it into a disc and roll it out to 1/4 inch thickness between two sheets of parchment paper.

Use a 2-inch cookie cutter to cut out shapes, placing them on a baking sheet lined with parchment paper.

Bake for 9 to 11 minutes until just firm. Allow to cool on the baking sheet for 2 minutes before transferring them to wire racks to cool completely.

Once cooled, spread about 2 tsp of the ganache filling on half of the cookies and top with the remaining cookies to create sandwiches.