Photo: Karen Thomas
Preheat the oven to 170°C, fan 150°C, gas 3. Lightly grease a 20cm square baking tin and then line it with baking paper.
Place 100g of the chopped white chocolate in a heatproof bowl. Add the butter and melt gently together over a pan of simmering water. Remove from the heat, stir until smooth and combined, and leave to cool slightly.
Add the sugar, vanilla extract and beaten eggs to the slightly cooled melted white chocolate and butter. Stir well to combine.
Sift both of the flours and Horlicks powder into the chocolate mixture and stir until smooth. Fold in 100g Maltesers and the milk chocolate and spoon into the prepared tin. Bake on the middle shelf for 35-40 minutes.
Leave to cool in the tin. Melt the remaining white chocolate in a heatproof bowl over a pan of barely simmering water. Meanwhile, lightly crush the rest of the Maltesers. Drizzle over a little of the the melted white chocolate. Then scatter over the crushed Maltesers, and finish with the remaining melted chocolate. Cool, then cut into squares.
Get ahead: make up to a day ahead. The blondies can be stored in an airtight container for up to 3 days, or frozen without the topping.
Kitchen secret: for a grown-up version, swap the milk chocolate for dark and decorate with dark chocolate curls.