For the cinnamon crumb: In a medium bowl combine the butter, brown sugar, flour and ground cinnamon, rubbing between your fingers and thumbs until it resembles irregular breadcrumbs. Then set aside.
For the sponge: Firstly, grease the base and sides., and line the base of a deep 8inch round cake tin, and set aside. Preheat the oven to 180C/160C Fan/Gas Mark 4.
In a large bowl, beat the butter and both sugars together with an electric hand-whisk until light and fluffy. Then mix in the eggs, one at a time, mixing well between each addition. Then in a small bowl, combine the flour and baking powder together. Mix this into the wet batter in two additions, mixing only until just combined. Peel, core and chop the apples into eighths, then stir this into the batter by hand.
To assemble: Pour half the batter into your prepared tin and spread out evenly. Then, for the cinnamon layer. In a small bowl, combine the sugar and ground cinnamon and sprinkle this evenly over the top of the batter. On top of this, spread the remaining batter evenly. Finally, sprinkle over the prepared cinnamon crumb in an even layer.
Bake for 1 hour until golden brown on top, and a knife/skewer inserted into the middle comes out almost clean. Leave the cake to cool in the tin for about 30 minutes, before turning out onto a cooling rack to cool completely.