1.Blend coffee with the water. Beat butter, sugar and egg in small bowl with electric mixer until combined. Stir in coffee mixture, sifted flours and cocoa, in two batches.
2.Knead dough on floured surface until smooth; roll dough between sheets of baking paper until 4mm thick. Cover; refrigerate 30 minutes.
3.Preheat oven to 180°C (160°C fan-forced). Grease oven trays; line with baking paper.
4.To make mocha custard; blend custard powder, sugar and chocolate with milk in small saucepan; stir over heat until mixture boils and thickens. Remove from heat, stir in liqueur. Cover surface with plastic wrap; refrigerate until cold.
5.Using 8.5cm square cutter, cut out 25 shapes from dough. Halve squares to make 50 rectangles; place on oven trays. Press three coffee beans on half of the rectangles.
6.Bake biscuits about 12 minutes. Cool on wire racks. Spread custard over plain biscuits; top with coffee-bean topped biscuits.
A square cutter will make it easier to make perfectly shaped biscuits. However, you can easily cut out well- shaped rectangles from the rolled out dough. Be fussy, roll the dough as evenly as you can, then use a ruler and a sharp knife to cut out the rectangles. Use Kahlua or Tia Maria in the custard.