Place the softened unsalted butter and 60 g icing sugar in a mixing bowl. Whisk until smooth and creamy.

Add the all-purpose flour to the butter mixture. Fold with a silicone spatula until a soft dough forms.

Add the milk to the dough if needed for it to come together. Fold just until evenly incorporated.

Place the dough between two sheets of parchment paper. Roll it into an even sheet about 4 mm thick.

Transfer the covered dough sheet to the refrigerator. Chill for 60 minutes until firm.

Preheat the oven to 180°C / 350°F. Line baking sheets with parchment paper.

Cut the chilled dough into matching cookie shapes using a Linzer cookie cutter. Cut windows in half of the cookies.

Arrange the cut cookies on the lined baking sheets. Bake for 8 to 10 minutes until the edges are lightly golden.

Move the baked cookies to a wire rack. Cool completely before filling.

Spread Nutella over the flat side of each whole cookie. Press a cut-out cookie on top to form sandwiches.

Place the filled cookies on a serving plate. Sift the remaining icing sugar over the tops.