This easy apple pie cake, with a layer of tender apples cooked with brown sugar and cinnamon and topped with a crunchy oat streusel, is perfect as a fall dessert, or for breakfast with a cup of coffee or tea.
To prepare cake: Preheat oven to 350°F. Coat a 9-inch-square baking pan with cooking spray and line with parchment paper, leaving a 2-inch overhang on 2 sides. Whisk flour, sugar, baking soda, salt and 1/2 teaspoon cinnamon together in a large bowl until combined. Add buttermilk, eggs, oil and vanilla; whisk just until smooth. Pour into the prepared pan and spread evenly. Set aside.
Combine apples, 1/2 cup brown sugar, 1 tablespoon butter, lemon juice and the remaining 1/2 teaspoon cinnamon in a medium skillet; bring to a simmer over medium-high heat. Simmer, stirring occasionally, until the apples are slightly tender and the sugar is dissolved, about 3 minutes. Spoon the apple mixture on top of the cake batter and drizzle with any sauce left in the skillet.
To prepare streusel: Place oats, butter, brown sugar and flour in a small bowl. Using your fingers, work the mixture together until coarse crumbs form. Sprinkle the streusel over the apple mixture.
Bake until the top is golden brown and a wooden pick inserted into the middle of the cake comes out clean, about 40 minutes. Let cool in the pan on a wire rack until completely cool, about 1 hour. Use the parchment paper handles to lift and remove the cake from the pan. Slice into squares.
To make ahead: Cover and store at room temperature for up to 1 day.