Place 40 g softened unsalted butter and 55 g icing sugar in a mixing bowl, then whisk until evenly combined and smooth.

Add 75 g all-purpose flour and 25 g dark cocoa powder to the butter mixture.

Add 1/4 teaspoon baking soda, then fold the dry ingredients into the butter mixture with a spatula.

Add up to 1 tablespoon milk a few drops at a time, then mix until the dough comes together.

Cover the dough and refrigerate it for 60 min, until firm enough to roll.

Place the chilled dough between or on sheets of parchment paper, then roll it into a thin even sheet.

Cut the dough into circles using a round cookie cutter, then transfer the circles to a parchment-lined baking tray.

Refrigerate the shaped cookie dough for 120 min to 180 min, until thoroughly chilled and firm.

Preheat the oven to 180°C (350°F), then bake the chilled cookies for 8 min to 10 min.

Remove the baked cookies from the oven and let them cool completely on the baking tray.

Place 50 g softened unsalted butter and 150 g icing sugar in a clean mixing bowl, then fold until a smooth cream forms.

Spread the cream into an even layer on parchment paper, then refrigerate until firm enough to cut.

Cut the chilled cream into circles using the round cookie cutter, matching the size of the cookies.

Place one cream circle on the underside of a cookie, then sandwich it with a second cookie.