Preheat the oven to 180℃ (160℃ fan)/350°F/gas mark 4. Grease and line a roasting tin with baking paper.
Meanwhile, put (75g) cocoa powder into a small bowl and slowly stir in 275ml (9½fl oz) boiling water, mix to a smooth paste. Set aside to cool slightly.
In a separate bowl, beat together the 200g butter,200g caster sugar and 175g light muscovado sugar until pale and fluffy then beat in 2 teaspoons vanilla extract.
Add 3 eggs one at a time beating well after each addition. Then fold in 100ml soured cream.
Sift 350g plain flour, 1 teaspoon baking powder and 1 teaspoon bicarbonate of soda together. Fold half the flour mixture into the butter and sugar mixture, followed by half the cocoa mixture. Repeat with the remaining flour and cocoa mixture until everything is combined to form a smooth cake batter.
Pour into the prepared baking tin and spread level. Bake for 25-30 mins or until risen and a skewer inserted into the middle of the cake comes out clean. Cool in the tin.