Preheat oven to 180°. Grease and line two oven trays with baking paper.

Beat butter, sugar, egg yolk and combine with coffee granules and hot water in a small bowl with electric mixer until smooth. Transfer mixture to a large bowl.

Add flour and cocoa together and knead the dough on a floured surface until smooth.

Roll table spoon level of  mixture into balls and press a little chocolate melt in to the center of each cookie. Bake for about 12 minutes. Cool cookies on trays.