1. Preheat the oven to 170C/325F/Gas3. Line a 24x24cm tin with baking parchment
2. Bring a pan of water to the boil. Put the plain and milk chocolate with the butter in a large bowl, place over the boiling water, switch off the heat and allow to melt
3. In a separate bowl, whisk the eggs and sugar together until light fluffy and creamy. Add the beer and whisk in thoroughly, followed by the melted chocolate and butter, beat in until all combined again.
4. Sieve the flour and cocoa powder into the eggs and fold in until thoroughly combined, then fold in the nuts and remaining white chocolate pieces.
5. Pour the chocolate mixture into the prepared tin. Place in the oven and bake for 20 minutes. The top should be firm but the inside should still feel soft, ie still have a slight wobble. It should still be gooey!.
6. Allow the brownies to cool in the tin
7. To make the frosting, beat the fettle and milk together with an electric hand mixer until soft. Gradually add the icing sugar, beating now with a wooden spoon to avoid a volcanic storm. Beat vigorously until soft
8. When the brownie has cooled, spread the icing over the top and cut the brownies into squares.