A hint of chilli and black pepper and the tanginess of mango make these biscuits subtly spicy.
First, make the jam. Place half the mango slices in a processor and blitz to a smooth purée. Pour the purée into a medium pan and add the sugar, lemon zest and juice, and chilli flakes. Place over a high heat and bring to the boil, then lower the heat to medium and cook for about 10 minutes, stirring occasionally, until the liquid has reduced and thickened.
Test for setting point – it should reach 105°C/221°F or jam-setting point on a sugar thermometer. (Alternatively, place a teaspoonful of jam on a saucer that has been stored in the freezer, then put the saucer in the fridge. After 1 minute, test the jam by running your finger through it. If the jam wrinkles it is ready.) If the jam isn’t ready yet, cook for another 1–2 minutes and re-test. Pour the jam into a bowl to set.
Meanwhile, place all the dry ingredients for the biscuits into the bowl of a stand mixer fitted with the beater attachment. Add the diced butter and mix on a slow speed until it forms fine crumbs.
Divide the mixture in half. Return one half to the mixer bowl and add 2 tablespoons of the milk. Mix on a slow speed for about 2–3 minutes, until you have a firm dough. Turn out the dough onto a lightly floured surface and knead gently until smooth. Wrap the dough in cling film and place in the fridge for 15 minutes, until firm.
Place the remaining dough mixture into the mixer bowl. Combine the remaining milk and the red food colouring in a small bowl until deep red, then add this to the dough. Mix, knead and chill as before.
Preheat the oven to 180°C/160°C fan/350°F/Gas 4.
On a floured work surface, roll out the uncoloured dough to a thickness of about 5mm. Use the 8cm cutter to cut out 24 discs, re-rolling the trimmings as necessary. Place the discs on the prepared baking sheets, spaced well apart. Bake for 15–17 minutes, until pale golden. Remove from the oven, leave to cool on the baking tray for 5 minutes, then transfer to a wire rack to cool completely.
Roll out and cut the red dough in the same way, then cut out the centre of each biscuit with the smaller 4cm cutter. Bake and cool as before.
Make up the royal icing. Put the egg white in the bowl of a stand mixer fitted with the beater attachment. Add 3 tablespoons of cold water and beat for 30 seconds, then add the sugar and beat on medium speed for about 10 minutes, until the icing turns pale and is aerated. Divide the icing into 2 small bowls and colour 1 with blue food colouring and the other with yellow. Put the blue icing in the piping bag with the No.2 nozzle, and the yellow icing in the piping bag with the No.1.5 nozzle.
Spread the uncoloured biscuits with plenty of mango jam, then top with the red biscuits. To decorate, pipe 2 parallel yellow lines across one side of the biscuits, then use the blue icing to pipe 3 dots between the lines. Leave to set.