Preheat oven to 350F. (regular bake setting/not fan assisted). Grease a 9-inch springform pan and line the bottom with a round of parchment paper.
Melt the 1/2 cup of butter ane set aside to cool.
In a medium bowl, whisk together the flour, baking powder, salt and cinnamon. Set aside.
Peel and core all 4 large apples. Cube 3 of the apples into 1/4-1/2-inch pieces and cut the 4th apple into thin slices. Set aside.
In a large bowl with an electric mixer or the bowl of a stand mixer with the paddle attachment, beat together the eggs and sugar, beating until it's light and syrupy, about 3 minutes. With mixer on low, add the cooled, melted butter and the vanilla. Beat to combine. With mixer on low, spoon in the flour mixture and mix until well combined.
Remove bowl from mixer. Fold in the diced apple until they are coated with the batter.
Spoon batter into prepared springform pan and level top. Scatter sliced apples over the top and press slightly into the batter.
Prepare the crisp topping by stirring together the oatmeal, brown sugar and cinnamon. Add the room temperature butter and use your fingertips to pinch the mixture together, until it form crumbles.
Sprinkle on the crumble topping evenly over the top of the cake.
Bake in preheated oven for 45-50 minutes, or until a tester inserted in the centre of the cake comes out clean.
Allow cake to cool in pan on a cooling rack for about 5 minutes, then run a knife around the edge and remove the outer ring of the springform pan. Allow to cool a bit longer, then run a knife under the parchment base and slide off the springform bake and onto a cooling rack. Gently remove the parchment from the bottom and discard. .
Serve warm or at room temperature, dusted with icing/confectioners' sugar.
Notes
Be sure to read the notes above this Recipe Card, where I share more detailedtips, variations and substitution suggestions for this recipe!