For the biscuits, put the flour, custard powder, baking powder, sugar and butter into a food processor. Pulse until the mixture resembles fine crumbs. Alternatively, rub the butter into the flour mixture using your fingertips.
Step 2
Add the egg and milk and pulse until the mixture just comes together. Tip on to a work surface, bring together with your hands, then wrap in clingfilm and chill for at least 30min or up to 1 day.
Step 3
Preheat oven to 180°C (160°C fan) mark 4. Line two baking sheets with baking parchment. Lightly flour a work surface and roll out dough until 3mm (⅛in) thick. Cut it into an even number of 3cm x 4.5cm (1¼in x 1¾in) rectangles, rerolling trimmings as needed. If you like, mark the rectangles with a diamond and a dot in each corner.
Step 4
Put rectangles on prepared sheets and bake for 13-15min until lightly golden. Cool on the sheet for 3min, then transfer to a wire rack to cool completely.
Step 5
Meanwhile, put the softened butter into a large bowl and sift over icing sugar and custard powder. Mix to combine (don't add water as it needs to be fairly thick). Set aside.
Step 6
When the biscuits are cool, sandwich together with some of the filling, using a piping bag if you want a neater finish.
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”!).
