Preheat the oven: Preheat oven to 356ºF (180ºC). Line a large baking sheet with parchment paper, set aside.
Cream Butter and Sugar: To a large mixing bowl or the bowl of a stand mixer with the paddle attached, add the softened butter, sugar and vanilla. Beat for 1 minute until pale and creamy, scrape down the sides.
Sift and beat: Sift over flour, custard powder and cocoa powder. Beat for 1 minute until it turns into crumbly dough. Turn off the beater and use your hands to bring it together into a soft dough.
Form the biscuits: Pick up 2 teaspoons of dough (weighing 15g/0.5 oz. each) and roll it into a smooth ball. Place on the lined baking sheet and using the back of a fork, lightly press it down to create that iconic texture. Repeat with remaining dough.
Bake: Bake for 10-12 minutes until they come off the paper easily. Remove from the oven and allow to cool on the sheet for 10 minutes. Transfer to a wire rack to cool completely.
Chocolate Filling
Prepare the filling: In a medium-sized mixing bowl, sift the sugar and cocoa powder then pour in the melted butter and one tablespoon of milk. Use a whisk to mix until well combined and it turns into thick buttercream, if it’s too dry, add the remaining one tablespoon of milk and mix.
Sandwich the biscuits: Sandwich your biscuits together by pairing off 2 biscuits. Then spoon about half a teaspoon to one teaspoon of filling onto the bottom of one biscuit. Place the second biscuit on top and gently press together to form the sandwich. Repeat with remaining biscuits.
Set the filling: Set aside until the filling is set, about 15-30 minutes. Enjoy!
Notes
Store: Keep the Yo Yo Biscuits in an airtight container at room temperature. They will stay fresh for up to 1 week.
Freeze: For longer storage, freeze the biscuits in an airtight container or a freezer-safe bag with layers separated by parchment paper. They can be frozen for up to 3 months.
Thaw: Thaw the Yo Yo Biscuits at room temperature for a few hours before serving.