Bring butter, egg, and sour cream to room temperature before starting.

Chop the chocolate finely for faster, smoother ganache.

In a medium bowl, whisk together the flour, cocoa powder, baking powder, baking soda, and salt until evenly combined.

In a stand mixer bowl, beat the softened butter with granulated sugar on medium speed until light and fluffy, about 3 minutes. Scrape down the bowl.

Beat in the egg, vanilla extract, and sour cream until just combined.

Reduce speed and slowly add the dry ingredients. Stop mixing as soon as the dough comes together to avoid overmixing.

Wrap the dough and refrigerate for 30 minutes to make it easier to roll and help prevent spreading while baking.

Roll out between parchment to 1/4 inch thickness and cut rounds or hearts with a cookie cutter, or scoop and flatten for rustic discs.

Transfer to the prepared baking sheet with even spacing.

Preheat the oven to 350°F (175°C).

Bake cookies for 9-11 minutes until set at the edges but still soft in the center.

Let cool on the sheet for 3 minutes, then transfer to a rack to cool completely.

Warm the heavy cream in a small saucepan until it just begins to simmer - do not boil.

Pour the hot cream over the chopped chocolate in a heatproof bowl. Let sit for 2 minutes. Gently stir until smooth.

Stir in butter and vanilla for shine and richer texture. Cool until thick but spreadable.

Pair similar-sized cookies. Pipe or spoon about 1 teaspoon of ganache onto the flat side of one cookie, then top with the matching cookie.

Press gently to spread the ganache to the edges. Repeat with remaining cookies.

Place assembled cookies on a baking sheet and refrigerate for 20-30 minutes so the ganache firms.

If desired, melt white chocolate and drizzle, or dust with powdered sugar. Add sprinkles for a Valentine look.

Allow any decoration to set before storing.