I've made a doll cake before in pretty much the same way you described, round cake with a pyrex bowl cake on top to be the dolls skirt. I used a madeira recipe, makes a really sturdy cake perfect for carving up.
You?ll need the following ingredients for an 8? Madeira Cake:
350g (12 oz) of Unsalted Butter
350g (12 oz) of Caster Sugar
350g (12 oz) of Self Raising Flour
175g (6oz) of Plain Flour
6 Eggs
Pre-heat the oven to 160°C/325°F/Gas 3. Grease and line the cake tin (pan) with baking parchment.
Cream the butter and sugar in a large mixing bowl until light, fluffy and pale. Sift the flours together in a separate bowl.
Beat the eggs into the creamed mixture, one at a time, following each with a spoonful of flour, to prevent the mixture curdling.
Sift the remaining flour into the creamed mixture and fold in carefully with a large metal spoon.
Transfer to the lined bakeware and bake, see above but everyone?s oven is different.. When the cake is ready it will be well risen, firm to the touch and a skewer inserted into the centre will come out clean.
Allow the cake to cool then, leaving the lining paper on, wrap the cake in foil or place in an airtight container for at least 12 hours before cutting, to allow the cake to settle (but you don't have to do this, if you haven't got time)
I wouldn't even start checking the cake til it's risen significantly, at least 40 min, I think mine took about 70 min but as it says, everyone's oven is different.
I didn't split my bowl cake and fill it with jam - in hindsight I would recommend you do, as it was a bit dry. Split them, spread on the jam, layer them up, then carve away, this cake can take it. Spread buttercream all over (250g unsalted butter to 1kg of buttercream, with a splash of boiling water to bind, and a bit of vanilla)
When I made the cake I wrapped the dolls legs in clingfilm, made a smallish hole about the width and depth of two fingers, then slid her in. Agree with whoever said to ice the cake then cut a cross over the hole, makes a neater finish.