Preheat the oven to 170�C and line a baking tray with parchment paper for later.
Cream the butter and sugar using a standing mixer until light and fluffy. Add in egg yolks and vanilla essence. Beat until well combined. Gradually add in the custard powder and cake flour in 3 phases, use the machine's lowest speed to "stiff" until well mixed.
Transfer the batter to a piping bag fitted with desired tip. Pipe the batter into a baking tray lined with parchment paper. Put a small diced cheny on top of each cookies.
Bake in the pre-heated oven at 170�C for about 15 minutes. Cool completely before baking in an airtight container.