Preheat the oven to 350°F. Grease a 9 inch springform pan and set it aside.

Begin by preparing the streusel topping. In a medium bowl, combine the flour, sugar, and rolled oats. Add the cold butter pieces and use your fingertips or a pastry cutter to work the butter into the mixture until it resembles coarse crumbs. The butter should be evenly distributed and the texture should look slightly sandy with small clumps. Place the streusel in the refrigerator while you prepare the cake batter.

In the bowl of a stand mixer or with a handheld mixer, cream together the butter and granulated sugar until the mixture is light and fluffy. This should take about two to three minutes and will create the base for a tender cake.

Add the eggs one at a time, beating well after each addition. The batter should become smooth and slightly glossy.

In a separate bowl, whisk together the flour, baking powder, cinnamon, and salt. Gradually fold the dry ingredients into the butter mixture, alternating with the milk or cream. Stir just until everything is incorporated and a thick batter forms.

Spoon the batter into the prepared springform pan and gently smooth the surface so that it is evenly distributed.

Scatter the sliced apples evenly across the top of the batter. There is no need to arrange them perfectly. Simply aim for a fairly even layer so that every slice of cake will include some fruit.

Remove the streusel topping from the refrigerator and sprinkle it evenly over the apples.

Place the cake in the oven and bake for 50 minutes to 1 hour. The top should be lightly golden and a toothpick inserted into the cake portion should come out clean without wet batter clinging to it.

Allow the cake to cool slightly in the pan before releasing the sides of the springform pan. This helps the cake set and makes slicing easier.