Preheat the oven to 190C/170C fan/gas 5. Butter 2 x 20cm loose-bottom cake tins at least 5cm deep. Line the bases with baking paper and butter this too.
Whisk the oil and caster sugar in a large bowl, then whisk in the egg yolks and milk, and fold in the carrot and banana. Sift in the flour and baking powder and stir in, followed by the spices and walnuts.
In another bowl, whisk the egg whites until stiff, then fold into the cake mixture in two goes. Divide evenly between the cake tins (weighing the mixture helps with this) and level.
Bake for 30-40 minutes until shrinking from the sides and a skewer inserted at the centre comes out clean. Run a knife around the edge of the cakes and leave to cool in the tin, then turn the cakes out and remove the paper.
For the frosting, cream the butter and icing sugar in a food processor until smooth. Transfer to a large bowl then fold in the cream cheese and vanilla extract until blended and smooth.
Spread one of the cakes with about a quarter of the frosting, sandwich with the other and use the remaining frosting to coat the top and sides. Decorate with baby marshmallows and place in the fridge to set for about an hour.
Makes 1 x 20cm cake.