Heat the oven to 190°C/170°C fan/gas 5. Line 2 baking trays with non-stick baking paper.
For the biscuits, using a rubber spatula, cream together the 200g butter and 75g icing sugar in a mixing bowl until light and fluffy. Add the flour, cornflour and custard powder, then gently mix in using a butter knife.
Tip onto a work surface lightly dusted with cornflour, then quickly and gently squeeze the dough together – don’t knead it (see tips).
Tear off a small piece, then shape and roll into a walnut-size ball and put on a baking tray. Repeat using the rest of the dough to give 30 balls, spacing them 3-4cm apart. Press down lightly on each ball with a fork dipped in cornflour to create thick biscuits. Bake for 15-20 minutes until golden. Cool the biscuits on the baking tray until you can handle them easily (see tips), then carefully transfer to a cooling rack.
Meanwhile, make the butter icing. Mix all the ingredients in a large bowl using a butter knife until creamy and smooth.
Once the biscuits have cooled completely, spread the flat side of one biscuit with plenty of icing, gently press another on top, then transfer to a tray. Once all the mo-yos are sandwiched, chill them in the fridge for 30 minutes to firm them up (see Make Ahead), then enjoy with a trad cuppa or trendy flat white.