This is a fantastic recipe to learn because it can be used all throughout the seasons. As long as you have the base of this dessert down, you can use it for whatever is at the market that's beautiful and fresh.
Apples: Peel, slice, and core apples; cut into ¼-inch slices. Heat skillet to medium and add butter to melt, followed by apples. Stir to coat apples with butter and cook, about 2 minutes. Sprinkle sugar on top of apples, then stir so sugar melts. When the sugar starts to melt, add the bourbon (optional) and bring it to a boil. Then remove from the heat and cool to room temperature, about 10 minutes. Use a a cake tester to check apples for softness.
Streusel: Preheat oven to 375 F. In a medium bowl, add pecans, oats, brown sugar, flour, and salt; mix until thoroughly combined. Work in butter so the mixture becomes crumbly and the butter soaks up all the dry ingredients. Set aside.
In the bowl of a stand mixer fitted with a paddle attachment, cream the butter and sugar on low speed, 30 seconds. Scrape down the bowl and paddle, then continue mixing on medium until smooth and creamy, 30 more seconds. With the mixer on low, add egg and vanilla. In a bowl, combine flour and baking powder. Then add half the flour mixture to the butter-sugar mixture, mixing just until combined; then add half of the milk. Repeat until all the flour and milk have been added. Turn the mixer off; scrape down the bowl and fold in the apple mixture by hand. Pour batter into a buttered springform pan, and add streusel in pieces on top. Bake for 30–40 minutes.
Remove cake from oven and use a cake tester to check for doneness; it should come out dry. Remove the springform pan and slice.