Rather like giant dominoes, these sandwich biscuits with a rich chocolate filling are a British classic loved by all ages. Homemade bourbons were always popular in the royal nursery.

MAKES 12 sandwich biscuits

FOR THE BISCUITS

115g soft butter

170g plain flour, sifted

1 tsp baking powder

3 tbsp unsweetened

cocoa powder

3 tbsp semolina or polenta

100g caster sugar

1 free range egg

1 tsp pure vanilla extract

FOR THE FILLING

60g butter

130g granulated sugar

2 tbsp unsweetened

cocoa powder

½ tsp pure vanilla extract

• To make the biscuits, preheat the oven to 200C/ 180C fan/gas 6. Line two baking trays with nonstick baking paper or silicone mats. In a food processor, combine the butter, flour, baking powder, cocoa powder and semolina or polenta and process until the mixture resembles fine breadcrumbs. Add the caster sugar, egg and vanilla and process just until the mixture comes together in a smooth dough. Do not overprocess or the biscuits will be very hard.
• Turn out the dough on to a lightly floured work surface and roll it out into a large rectangle about 6mm thick. Using a ruler, cut the dough into rectangles measuring 5cm x 7.5cm. Lift away any scraps and set aside. Using a fork, prick each rectangle to make an attractive pattern of dots. Then, using an offset spatula, carefully transfer the rectangles to the prepared trays, spacing them about 2.5cm apart. Gather up the scraps, press together, reroll, cut out more rectangles, decorate them with dots and add them to the trays. You should have 24 rectangles.
• Bake the biscuits for 8-10 minutes until firm to the touch. Leave to cool on the trays on wire racks for 5 minutes, then transfer to the racks and leave to cool completely.
• To make the filling, in a small saucepan, melt the butter over low heat. Remove from the heat, add the granulated sugar, cocoa powder and vanilla, and stir to dissolve the sugar and cocoa powder, mixing well.
• When the biscuits are cool, turn 12 of them bottom side up on a work surface. Spread a generous amount of the filling on each overturned biscuit, dividing it evenly and spreading it to the edge. Top with a second biscuit, bottom side down, and press lightly to secure. The biscuits will keep in an airtight container at room temperature for up to four days.