Vanilla-Filled Chocolate Cookies

For the cookies:
Preheat the oven to 190C/375F/Gas 5. Line three baking trays with parchment paper. Set aside.

Sift flour, cocoa powder, baking soda, baking powder, and salt into a bowl. Set aside.

Using a handheld or stand mixer fitted with the paddle attachment, cream butter and sugar together for 5 minutes until light and fluffy. Add egg and beat on medium speed until just incorporated. Lower the speed and add the dry ingredients gradually until dough is well combined.

Using a small scoop or teaspoon, drop round dough balls (weighing about 15 grams or 1/2 ounces each) onto the prepared baking trays about 5cm (2 inches) apart. Press the bottom of a glass into a bowl of sugar and use to flatten the cookie dough to about 1/4 inch thick.

Bake for about 10 minutes until the cookies are firm. Transfer to a wire rack to cool completely.

For the vanilla filling:
Using a handheld or stand mixer fitted with the paddle attachment, beat butter until light and fluffy. On low speed, carefully add icing sugar. Turn up the speed and beat for 3 minutes until light and fluffy. Add the vanilla and continue to beat for at least 2 more minutes.

Transfer the filling to a piping bag and pipe about 1 tablespoon of the filling onto one of the cookies, leaving a little space around the edges. Cover with another cookie and gently press together so that the filling squeezes out near the edges of the cookies. Repeat with all the cookies.