Marbled chocolate and banana mixtures make a deliciously moist cake, which is packed full of flavour. They are also a great way to use up over-ripe bananas.
Heat the oven to 180°C/160°C fan/350°F/Gas 4. Beat the butter and sugar together in a stand mixer fitted with the beater, on medium speed until pale and fluffy. Add the eggs, little by little, until well incorporated.
In a separate bowl, stir together the bananas, vanilla and yogurt. Combine the flour and bicarbonate of soda in another bowl. Alternately, fold spoonfuls of the banana mixture and flour mixture into the mixer bowl with the egg and butter mixture until you have a smooth batter.
Spoon one third of the batter into another bowl and stir in the cocoa powder and milk until fully combined. Stir the chocolate chips into the vanilla cake mixture. Place each mixture into a large piping bag, if using, and snip the end of each to make 2cm holes.
Pipe alternate grooves of the bundt tin with the cocoa and vanilla cake mixtures, piping the remaining mixture in the centre. Alternatively, place alternate spoonfuls of each mixture into the tin to give a marbled effect. Then, level the surface with a palette knife and bake for 50–60 minutes, or until a skewer inserted into the sponge comes out clean. Leave the cake to cool in the tin for 15 minutes, then turn out onto a wire rack to cool completely.
Make the toffee sauce. Pour 1 tablespoon of water into a small pan, add the caster sugar and stir over a low heat until dissolved. Then, increase the heat and boil for about 5 minutes, shaking the pan occasionally (but don’t stir) until you have pale golden syrup. Stir in the butter and whisk well. Take the pan off the heat and stir in the double cream and mix well. Transfer the bundt to a serving plate and drizzle the sauce down the grooves. Finally, sprinkle over the walnuts and dot with the banana chips.