Preheat oven to 190°C (170°C fan) mark 5. To make the biscuits, using a handheld electric whisk, beat together butter, icing sugar and vanilla in a medium bowl until light and very fluffy. Sift in the plain flour and cornflour, and beat until just combined.
Step 2
Split mixture in half. Into one bowl, sift the cocoa powder and beat to combine. Line 2 large baking sheets with baking parchment.
Step 3
Fill one side of a piping bag (fitted with a 1cm closed star nozzle) with the plain mixture, using your hands to shape it. Squash the chocolate mixture into the other side of the piping bag.
Step 4
Pipe 2cm swirls on to the lined sheets, spacing apart. The mixture will be very stiff – it helps if you seal the piping bag and squeeze from the middle rather than the top. Chill for 20min until firm. Bake for 15min until lightly golden. Cool on trays for a few minutes, then transfer to a wire rack to cool completely.
Step 5
To make the filling, in a large bowl, beat together butter, icing sugar and milk with a handheld electric whisk. Beat in the grated chocolate. Pipe or spread chocolate over bases of half the biscuits, then sandwich together with remaining biscuits. Serve.
Get Ahead: Keep sandwiched biscuits in an airtight container at room temperature for up to 3 days.
An experienced and highly skilled team of food writers, stylists and digital content producers, the Good Housekeeping Cookery Team is a close-knit squad of food obsessives. Cookery Editor Emma Franklin is our resident chilli obsessive and barbecue expert, who spends an inordinate amount of time on holidays poking round the local supermarkets seeking out new and exciting foods. Senior Cookery Writer Alice Shields is a former pastry chef and baking fanatic who loves making bread and would have peanut butter with everything if she could. Her favourite carb is pasta, and our vibrant green spaghetti is her weeknight go-to. Lover of all things savoury, Senior Cookery Writer Grace Evans can be found eating crispy corn and nocellara olives at every opportunity, and will take the cheeseboard over dessert any time (though she cannot resist a slice of tres leches cake). With a wealth of professional kitchen know-how, culinary training and years of experience between them, they are all dedicated to ensuring every Good Housekeeping recipe is the best it can be, so you can trust they’ll work (and if they don’t – we’ll have the answer for why*) every time (*90% of the time the answer is: “buy an separate oven thermometer”!).