Preheat the oven to 180 C / Gas 4. Grease and flour muffin tin.
Melt the butter and allow to cool slightly.
In the bowl of a mixer, sift in the sugar and flour. Mix and add the melted butter and dulce de leche. Beat until well combined. Add eggs, one at a time, beating constantly. Beat until smooth. Spoon mixture into prepared tin.
Bake in preheated oven for 12 minutes or until golden brown. Remove from oven and unmould while cakes are still hot.
You can use shop-bought dulce de leche, if you can find it. Another option is to use tinned shop-bought caramel, the kind used for banoffee pie.