Preheat oven to 190 C / Gas 5. Grease a 25cm (10 in) loose bottomed cake tin.
Melt butter in a pan over a medium heat until just runny and pour into a large bowl. Add eggs, sugar and almond essence and beat well until mixed. Fold in flour and baking powder and spread just under 2/3 of the mixture in the prepared cake tin.
Peel, core and slice apples and arrange roughly on top of the mixture. Spread the remaining mixture over the apples. It will be difficult to get it smooth, but it will even out during baking.
Bake for 1 1/2 hours until apple is tender when prodded with a skewer. Loosen the sides of the tin and push the cake out. Dust generously with icing sugar. Serve warm or cold with lots of lightly whipped or thick cream.