Heat the oven to 180C/160C fan/gas 4. Scatter the oats over a baking sheet and toast in the oven for 8-10 mins, mixing halfway, until golden. Leave to cool completely.

Tip the flour, cornflour and sugar into the bowl of a food processor and pulse to combine. Add the butter and pulse to the texture of breadcrumbs. Pour in the egg yolks and pulse again until the mixture is moist and begins to come together. Add a splash of water if needed.

Turn the dough out onto a work surface and scatter over the cooled toasted oats. Work them into the dough, using your hands, bringing it together into a ball. Split the dough in half and form into two rough squares. Wrap each one in baking parchment and chill for 1 hr.

Heat the oven to 180C/160C fan/gas 4. Remove the dough from the fridge and roll each piece into a rough 30 x 30cm square. Trim away any rough edges. Slice into 3cm wide strips vertically, then turn the dough and slice into 7cm-long biscuits. You should have 50 in total. Will keep frozen for up to three months.

Transfer the biscuits to two lined baking trays, spacing them apart slightly, then bake for 12-14 mins or until the edges are lightly golden. Leave to cool on the tray.

Meanwhile, make the buttercream. Beat the butter using a stand mixer or electric beaters on high speed until very pale, about 7 mins. Add the malt powder and vanilla and beat again. Sift in the icing sugar and combine on a low speed. Once fully combined, whizz on high until silky and thick. Transfer to a piping bag.

Turn a biscuit over so you can pipe on the side that was face-down on the tray. Pipe the icing over, then lightly press another biscuit on top. Repeat with the remaining of the biscuits and buttercream. Best served on the day of baking, but will keep for three days in an airtight container.