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Line a 20cm square brownie tin with baking parchment. Put the butter, sugar, condensed milk and whole milk into a pan and heat gently for a few minutes until the butter is fully melted.
Turn the heat up and simmer for 20-30 mins, stirring regularly to prevent the mixture sticking to the bottom of the pan, until thick and darker in colour.
Remove from the heat and use an electric whisk to beat the mixture for 5 mins. Stir in the vanilla, then tip the mixture into the prepared tin. Level the top and leave to cool completely before cutting into small squares.