Add butter and sugar to a mixing bowl and using an paddle mixer, beat until pale and fluffy.
Add vanilla extract and mix through
Add plain flour and custard powder and continue mixing until a soft dough forms.
Roll dough into 24 balls and place on the baking tray. Each ball will be around 2/3 size of a golf ball. Space the balls apart leaving room to expand. Using a fork gently flatten each ball. Note: The dough will expand as it bakes.
Place in oven and bake for about 15 minutes until golden. Remove from oven and set aside to cool completely.
Add butter and icing sugar to a bowl and beat until pale and fluffy.
Add raspberries and continue mixing until fully combined.
Spread buttercream and sandwich between a pair of biscuits creating 12 yo-yo biscuits in total. Store in refrigerator.