1. Preheat the oven to 350°F with a rack set in the middle position.
2. Grease and flour a 9-inch round cake pan.
3. Set aside.

Make the streusel topping:

1. In a medium bowl, stir together ½ cup flour, oats, ¼ cup dark brown sugar, ½ teaspoon ground cinnamon, and ¼ teaspoon salt.
2. Add 4 tablespoons butter and, using a fork or your fingers, blend until the mixture resembles small peas.

Make the cake:

1. In a large bowl, whisk 1¾ cups flour, baking powder, 1½ teaspoons ground cinnamon, baking soda, and ¼ teaspoon salt until blended.
2. Add apple, pecans, and currants and mix until ingredients are coated with the flour mixture.
3. Set aside.
4. In the mixing bowl of the stand mixer fitted with the flat beater attachment, beat ¾ cup light brown sugar, granulated sugar, and 8 tablespoons butter on medium (5-6) speed until creamy.
5. Add eggs and vanilla.
6. Beat until blended.
7. Add sour cream and beat until blended. Reduce the speed to low (1-2).
8. Slowly add the flour mixture.
9. Pour the batter into the prepared cake pan and spread evenly.
10. Sprinkle the streusel topping over the batter.
11. Bake until a cake tester inserted into the center comes out clean and the top is browned, 45-50 minutes.
12. Cool in the pan on a wire rack for 10 minutes.

Make the glaze, if desired:

1. In a small bowl, whisk together confectioners’ sugar, milk and ¼ teaspoon ground cinnamon until smooth.
2. Drizzle over cooled cake.

Pro Tip:
If serving the cake on a serving platter or cake plate, line the bottom of the pan with parchment paper.