Put each flour into its own mixing bowl and add a sachet of yeast and ½ tsp salt to each bowl. Stir the caster sugar into the white flour, and the muscovado and spices into the brown flour.
Put the milk and butter in a small pan and warm gently until the butter has melted. Make a well in the middle of each flour mix and pour half the milk mixture into each. Add 1 egg to each, and mix to a soft, sticky dough – if you have a table-top mixer or food processor with a dough hook, use that to mix, as the dough is quite sticky.
Turn each dough onto a lightly floured surface and and knead each for 5 mins until smooth (or knead in your mixer). Put back into their bowls, cover loosely with oiled cling film and leave somewhere warm until the dough has doubled in size. Lightly dust a 900g loaf tin with flour.
Turn out the doughs and knead each for 30 secs more. Roll out each dough thinly so that its width is the same as the length of the loaf tin, but roll it as long as you can – the longer it is, the more ‘swirls’ you’ll get. Melt the 2 tbsp butter and brush the spicy malted dough all over, then scatter with the raisins and extra 1 tbsp of muscovado sugar. Lift the white dough on top and press together. Brush the top with the remaining melted butter, then roll up from one of the ends that matches the length of the tin. Lift the roll into the tin and cover loosely with oiled cling film. Leave somewhere warm for 20-30 mins until the dough has risen nicely above the top of the tin.
Heat oven to 200C/180C fan/gas 6 and bake for 20 mins, then turn down to 180C/160C fan/gas 4 and bake for 20 mins more. Turn out the loaf and check it sounds hollow when tapped on the base – if not, pop back in the tin and bake for another 5 mins. Cool on a wire rack, then slice, toast and butter or eat cold.