Using an electric mixer, beat butter until smooth and fluffy. Sift icing sugar and cocoa over butter and beat in. Combine milk, coffee granules and vanilla and heat in microwave on HIGH for 8 seconds, stirring to dissolve coffee. Add to butter mixture along with egg yolk.
Stir together flour, baking powder and salt; stir into butter mixture, blending well. Shape dough into a disc, wrap and chill 20 min.
Preheat oven to 325°F (160°C). Line 2 baking sheets with parchment paper. On a surface lightly dusted with icing sugar, roll dough 1/8-inch (3 mm) thick. Cut into 48 1-1/2 in. (4 cm) shapes (fluted circles are pretty). Cut a 1/2-inch (1 cm) hole in the centre of half the cookies. Transfer carefully to baking sheets and bake 12 to 15 min., or until cookies can be lifted easily away from the parchment. Let cookies cool on baking sheets.
To assemble, dust the cookies with holes with icing sugar (top cookies). Spoon a teaspoonful of jam onto bottom cookie and place a dusted cookie on top, pressing gently to secure.