1/2 cup unsalted butter
1/2 cup granulated sugar
1 egg yolk
2 tbsp sour cream
1/2 tsp vanilla
1 cup all-purpose flour
1/3 cup cocoa powder
1/2 tsp baking soda
2 tbsp unsalted butter, softened
1 cup icing sugar
1 tbsp whipping or 10% cream
In large bowl, beat butter with sugar until fluffy; beat in egg yolk. Beat in sour cream and vanilla. In separate bowl, whisk flour, cocoa powder, baking soda and salt; stir into butter mixture in 2 additions to make smooth dough.
Roll by 1 tsp (5 mL) into balls; arrange 2 inches (5 cm) apart, on parchment paper-lined baking sheets. Flatten into 1-inch (2.5 cm) rounds. Bake in 350 F (180 C) oven until firm to the touch, 5 to 6 minutes. Let cool on pan on rack for 2 minutes, transfer to racks and let cool.
Filling: In small bowl, beat butter until fluffy; beat in sugar and cream until smooth. Spread 1/2 tsp (2 mL) over bottom side of half the cookies; sandwich, bottom side down, with remaining cookies. (Make Ahead: Refrigerate in airtight container for up to 3 days or freeze for up to 2 weeks.) Makes about 40 cookies.
Source: Canadian Living, Home for the Holidays, 2008