Ingredients (Makes 16)
Preheat the oven to 180°C, 350°F, Gas 4. Lightly butter and base line a 20cm/8in square cake tin with greaseproof paper.
Place the chocolate in a heatproof bowl set over a pan of gently simmering water and melt gently (do not allow the base of the bowl to touch the water). When the chocolate has melted, remove the bowl from the heat and leave to cool.
Meanwhile, place the butter in a large mixing bowl and beat until soft and creamy using a wooden spoon or an electric mixer. Add the sugar and vanilla essence and continue beating until the mixture is soft and fluffy. Gradually beat in the eggs.
Sift in the flour, cocoa and salt, then pour in the cooled melted chocolate. Stir everything together until thoroughly combined. Finally, stir in the walnut pieces, marshmallows, if using, and Maltesers. Spoon the mixture into the prepared tin and level the top.
Bake for 30-35 minutes, until a skewer inserted into the middle of the brownie cake comes out clean. Leave in the tin for 10 minutes then turn out and cut into 16 squares. Eat warm or at room temperature.