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Butter 2 x 20cm non-stick sandwich tins and line with circles of baking paper. Heat oven to 180C/fan 160C/gas 4. Using electric hand beaters, beat the butter, 3 tbsp peanut butter, eggs, sugar and yogurt together until smooth and creamy. Fold in the flour, then split the mix between the tins, using a spatula to get every last bit. Bake for 30 mins until risen and golden.
Meanwhile, toss the salted peanuts and icing sugar with ½ tsp water until well coated and claggy. Spread over a non-stick baking sheet. When the cakes are ready, leave them to cool for 5 mins in the tin, then turn onto a cooling rack to cool completely. Turn the oven up to 200C/fan 180C/gas 6, then roast the nuts for 10 mins, tossing them a couple of times until golden and coated in a shiny caramel. Tip onto a lightly buttered plate. Make the topping while you wait: melt the chocolate and milk together in a pan over simmering water or microwave on High for 1 min, then stir to melt. Leave to cool.
Put one of the sponges onto a plate, spread with 2 tbsp peanut butter (or more), then dot with the dulce de leche and spread with a palette knife. Sandwich the second sponge on top, then spread with the chocolate topping. Scatter with the caramelised peanuts to serve.