1. Preheat the oven to 180C conventional or 160C fan-forced. Line two baking trays with baking (greaseproof) paper.
2. Into a stand mixer fitted with the paddle attachment (or into a large mixing bowl if using electric beaters), sift together the flour, icing sugar, custard powder and salt. Mix on low speed to combine.
3. Whilst mixing on low speed, gradually add the butter until the mixture resembles wet sand.
4. Add the vanilla extract and continue mixing until the mixture comes together into a cookie dough.
5. Roll heaped teaspoons of the cookie dough into balls (see note 2) and place on the lined baking trays leaving space in between (approximately 3cm) to allow for spreading. Using a fork, flatten the balls slightly – you should be able to see indentations from the fork prongs.
6. Bake for 18 to 20 mins (swapping the trays halfway through the baking time if using two shelves in the oven). The cookies should look dry and only just starting to colour (start checking at the 18 minute mark).
7. Cool on the trays for 5 minutes before carefully transferring to a wire rack to cool completely.
8. Meanwhile, make the buttercream filling – link to recipe is above (see note 1). 

Assembling the Yo Yo Biscuits

1. On a clean surface, place half of the cookies flat-side up. Using a piping bag or small offset spatula, pipe/spread the buttercream filling onto the cookies. I like to be generous with the frosting so I tend to use about a tablespoon of the filling for each cookie sandwich. Sandwich with the remaining cookies.
2. Enjoy!