1 Pre-heat the oven to 170°C/fan oven 150°C/ Gas Mark 3. Grease a 23cm (9 inch) shallow square cake tin with a little Trex.
2 Put the pieces of Trex, butter and chocolate into a large saucepan and heat gently, stirring with a wooden spoon until melted, taking care that the mixture doesn’t get too hot.
3 Remove from the heat and cool for a few minutes.
4 Whilst the chocolate mixture is cooling, whisk the eggs and sugar together in a large bowl using a hand-held electric mixer. Add the melted chocolate mixture, stirring it in with a large metal spoon. Sift in the flour and baking powder and fold it in gently.
5 Tip the mixture into the prepared tin. Bake for 35-40 minutes until firm. Test that the cake is cooked by inserting a fine skewer into the middle – there should be a few crumbs sticking to the skewer. Place the tin on a wire rack and cool for an hour, then cut the brownies into squares, remove from the tin and cool completely on the rack.
Recipe courtesy of Trex