- 125g unsalted butter, softened, plus extra for greasing
- 170g self-raising flour
- ½ tsp bicarbonate of soda
- ½ tsp salt
- 200g caster sugar
- 3 large eggs
- 1 tsp pure vanilla extract
- 150g plain yogurt
- 40g cocoa powder
- 60ml boiling water
For the white chocolate icing
- 150g icing sugar
- 2 tbsp milk
- 75g good-quality white chocolate (e.g. Valrhona), broken into pieces
Line the base and sides of a 900g loaf tin with butter and baking paper. Preheat the oven to 180°C/gas 4.
Sift together the self-raising flour and bicarbonate of soda into a bowl, and set aside.
In another bowl, beat the butter and sugar until very pale and fluffy. Crack the eggs into a jug and break up the yolks with a fork, then stir in the vanilla and slowly add to the butter and sugar, beating until fully incorporated.
Add half of the sifted flour and bicarbonate of soda and mix just to combine. Then mix in half of the yogurt. Add the remaining flour mixture and beat to combine, before adding the remaining yogurt. Scrape the bottom of the bowl with a rubber spatula to pick up any butter that may be trapped at the bottom. Give the batter a final mix.
Separate a third of the batter into a clean bowl. Whisk the cocoa powder and boiling water together with a hand whisk or fork and add to this portion of batter, folding it through with the spatula.
Spoon dollops of batter into the prepared loaf tin, alternating spoonfuls of the cocoa-flavoured batter and the plain batter, to form a chequerboard-like pattern. Drag a knife through the batter in a swirling motion, to create a marbled effect, but do not over-swirl; a few strokes will be enough.
Bake the loaf cake for 45–55 minutes until it springs back to the touch and a skewer inserted into the centre comes out clean. Remove from the oven and allow the cake to cool for 10–15 minutes before turning it out onto a wire rack to cool.
While the cake is cooling, prepare the icing. Melt the chocolate in a heatproof bowl set over a pan of simmering water, then remove from the heat and allow to cool. In a separate bowl, whisk together the icing sugar and milk. Add the cooled, melted white chocolate and whisk until smooth and uniform.
Pour over the cake right away so that it does not form a skin, allowing it to drip over the sides. The cake will keep in an airtight container for up to a week.