For the peanut frosting
Preheat the oven to 180°C/350°F/Gas mark 4. Grease a 30 x 23cm/ 12 x 9in baking tin.
Sift the flour, cocoa, baking powder and salt into a bowl. Cream the butter and sugar, then beat in the eggs and the vanilla essence. Stir in the dry ingredients, and beat until smooth. Fold in half of the nuts.
Spoon the mixture into the greased tin, and bake for 30 minutes.
To make the peanut frosting, cream the butter and gradually mix in the icing sugar until light and fluffy. Combine the peanut butter with the boiling water and stir into the mixture.
When the brownies are cooked, spread with the frosting and decorate with the remaining nuts.