Serving: Makes about 2 dozen 2-inch (5 cm) sandwich cookies
½ cup + 2 Tbsp (155 ml) unsalted butter at room temperature
½ cup (125 ml) sugar (caster sugar)
1 large egg yolk
1 tsp (5 ml) vanilla extract
1 ¼ cups (310 ml) all-purpose flour (plain flour)
¼ cup (60 ml) cocoa powder
¼ tsp (1 ml) baking powder
¼ tsp (1 ml) salt
¼ cup (60 ml) unsalted butter at room temperature
¾ cup (185 ml) icing sugar, sifted
2 Tbsp (30 ml) milk
½ tsp (2 ml) vanilla extract
1. Beat the butter and sugar until light and fluffy. Beat in the egg yolk and then beat in the vanilla.
2. In a separate bowl, sift the flour, cocoa powder, baking powder and salt. Add this to the butter and stir just until blended. Divide this into 2 discs, wrap and chill until firm, at least an hour.
3. Preheat the oven to 350°F (175°C) and line 2 baking trays with parchment paper. Give the dough one or two kneads on a floured work surface to soften it (this will prevent the dough from cracking when rolling) and then roll it out to just less than 1/4-inch (6 mm) thick. Cut out cookies using a 2-inch (5 cm) cookie cutter and lift onto the baking trays. Any scraps can be re-rolled, and if the dough gets too soft when rolling, simply chill it for 15 minutes before rolling again.
4. Bake the cookies for about 8 minutes, until they lose any shine to them. Allow the cookies to cool on the tray before removing to fill.
5. For the filling, beat the butter with ½ cup (125 ml) of the icing sugar until fluffy. Beat in the milk and vanilla, and then beat in the remaining ¼ cup (60 ml) of icing sugar.
6. To assemble the sandwich cookies, spread a generous teaspoonful of the filling on the bottom side of one cookie and press the bottom side of a second cookie to cover the filling. Repeat this until all the cookies are sandwiched. Store the cookies in an airtight container for up to 3 days.