The heavenly combination of peanut butter and chocolate is well documented and this cake is every bit as delicious as you’d expect it to be. Topped with peanut and chocolate ganache, the result is, well, the icing on the cake.
1.Preheat oven to 180°C/350°F. Grease a deep 20cm (8-inch) square cake pan; line base and sides with baking paper.
2.Stir cocoa and the water in a small heatproof jug until cocoa dissolves. Cool 10 minutes.
3.Beat butter, sugar and extract in a small bowl with an electric mixer until pale and fluffy. Beat in eggs, one at a time, then cocoa mixture and peanut butter until combined (mixture may look curdled at this stage). Beat in sifted dry ingredients and buttermilk, in two batches, until smooth. Spread mixture into pan.
4.Bake cake for 1¼ hours or until a skewer inserted into the centre comes out clean. Stand cake in pan to cool slightly.
5.Turn onto a wire rack to cool.
6.For peanut butter ganache, place chocolate and peanut butter in a medium heatproof bowl. Heat cream in a small saucepan until almost boiling; pour over chocolate mixture, whisking until smooth. Cover; refrigerate for 1 hour, stirring occasionally until mixture is chilled and thickened.
7.Beat cold peanut butter ganache with an electric mixer until pale and fluffy. Spread ganache on top of cooled cake; top with peanut-butter-filled chocolate.
The cake can be made up to 3 days ahead; store in an airtight container at room temperature. The peanut butter ganache is best made on the day of serving.