Irish Apple Cake

Combine 1/2 cup flour, 1/2 cup sugar, oats, 1/4 teaspoon salt, and 1/4 teaspoon cinnamon in a bowl for the streusel. Using a fork or pastry cutter, cut butter into the mixture until it is fully incorporated and has the texture of coarse bread crumbs, about 2 minutes. Chill in the refrigerator until ready to use. 

Preheat the oven to 350 degrees F (175 degrees C). Grease a 9-inch round cake pan with cooking spray.

Combine 1/2 cup butter, 1/4 cup granulated sugar, and brown sugar in a medium bowl for the cake batter. Beat at medium speed, until lightened in color and fluffy, 3 to 4 minutes. Beat in eggs, 1 at a time, on low speed, just until blended. Beat in vanilla extract.

Combine 1 1/4 cups flour, baking powder, 1 teaspoon cinnamon, and 1/4 teaspoon salt in a small bowl. Add dry ingredients to the mixing bowl and mix on low speed until fully incorporated, 1 to 2 minutes. Add 3 tablespoons milk and beat on low speed until combined, about 45 seconds.

Spread cake batter evenly in the prepared pan.

Arrange apple slices evenly on top of batter, overlapping as needed.

Sprinkle apples with streusel.

Bake in the preheated oven until a toothpick inserted into the center comes out clean and streusel is golden brown and crisp, 40 to 50 minutes. Let cake cool in pan on a wire rack 10 minutes. Remove from pan to wire rack and let cool completely, 30 to 45 minutes.

Whisk together powdered sugar, 1 teaspoon cinnamon, and 2 teaspoons milk in a small bowl until smooth.

Drizzle glaze over cake before serving.