Photograph: Peter Cassidy
Preheat the oven to 170°C, fan 160°C, gas 3 and line two large baking sheets with baking parchment. For the biscuits, in a large bowl – using an electric hand mixer – beat together the butter and icing sugar until pale. Sift in the flour and cornflour, then whisk again.
Transfer half the mixture to another bowl and whisk in the cocoa powder. Next, take dessertspoonfuls of both mixtures (keeping them separate) and roll into balls. Place each ball on to a baking sheet then flatten each one into a disc with the back of a fork that you have dipped in water.
Bake on the centre shelf and shelf below for 12-15 minutes, until the vanilla biscuits are lightly golden (swap trays halfway through). Transfer to a rack until cooled completely. For the fillings, mix all the ingredients for each filling in separate bowls, then chill.
Sandwich the pale biscuits with the chocolate mixture and vice versa. Chill for at least 30 minutes before eating.
Get ahead: the cooked biscuits can be frozen (before filling).