Preheat oven to 180 degrees C and grease a 20cm springform cake tin.
In a large bowl, mix together the flour, brown sugar, mixed spice and baking powder, then rub the butter into the flour mixture until it resembles coarse breadcrumbs. Spoon almost half this mixture over the base of the prepared tin.
Whisk the bicarb soda, egg and nutmeg into the milk, then add the milk mixture to the remaining flour/butter mixture. Stir well.
Pour the batter over the crumbs in the tin, then sprinkle the white sugar over the top.
Bake for 1 hour and 10 minutes, or until golden brown and springy to the touch.
Cool in the tin for a few minutes, then turn out and cool thoroughly on a wire rack.
Dust with icing sugar and serve with a dollop of good thick cream.