In a medium mixing bowl, whisk together flour, cocoa powder, baking soda, baking powder and salt. Set aside.

In a separate large mixing bowl, using a handheld electric mixer, beat butter with sugars, red food coloring and vanilla until butter is fluffy, 3 - 4 minutes.

Add egg and beat until combined, then add buttermilk and mix until thoroughly combined.

Add flour mixture to butter and sugar mixture and mix on low speed until you no longer see big pockets of flour, about 2 minutes. Use a rubber spatula (a dark one if you have it - white will stain) to scrape the bottom and sides of the bowl. Fold cookie dough a few times to make sure everything is well combined.

Use a small cookie scoop (1 tbsp. size) to portion 36 balls of dough onto a parchment-lined baking sheet.

Roll cookie dough in red sanding sugar, then return to baking sheet and cover with plastic wrap. Refrigerate dough for 1 hour.

Preheat oven to 325°F with convection fan // 350° F without fan // 180°C // Gas mark 4.

Line 3 baking sheets with parchment paper and place 12 balls of dough on each sheet. (Bake one sheet at a time for best results. Keep cookie dough in the fridge until ready to bake)

Bake for 12 minutes. Remove from oven and let cool on baking sheet for 10 minutes before moving to cooling rack to cool completely.

While cookies are cooling, make peppermint buttercream.

When cookies have cooled completely, line them up in pairs, flipping over the bottom of the sandwich cookie.

Using a piping bag, or a small spatula, fill cookies with about 2 tablespoons of peppermint buttercream.

Place top cookie on buttercream and lightly press down to secure in place.

Store in an airtight container at room temperature for 2 days, or refrigerate for up to 5 days.