Mary Berry's chocolate and vanilla marble loaf cake

METHOD Preheat the oven to 160°C/325°F/gas mark 3. Lightly grease a 900g (2lb) loaf tin. Line with a wide strip of non-stick baking parchment to go up the wide sides and over the base. Measure the butter, sugar, flour, baking powder, eggs, milk and vanilla extract into a large bowl and beat with a …