Preheat the oven to Gas Mark 3, 170ºC, fan 150ºC. Grease a deep 23cm (9in) square cake tin and line with nonstick baking paper.
Melt the butter and chocolate in a heatproof bowl set over a saucepan of simmering water. Stir until smooth. Remove from the heat, add the sugar, eggs and vanilla and stir until combined. Add the flour and a pinch of salt and mix until combined.
Pour into the tin and bake for 40-45 minutes or until a skewer inserted into the centre comes out clean. Remove from the oven, then cool in the tin. Invert onto a board and remove the baking paper.
To make the frosting, mix the butter, icing sugar, and milk in the bowl of an electric mixer on medium speed for 2 minutes or until creamy and smooth. Spread the frosting evenly over the brownie.
Using a heart-shaped cutter, press out hearts from the brownie. Use a peeler to shave the white chocolate into curls and sprinkle over the hearts.
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