In a medium bowl, combine topping ingredients with a fork until crumbly. Break up any large pieces. Set aside.
In a small bowl, combine filling ingredients. Set aside.
In the bowl of a stand mixer, cream together butter and sugar. Mix in egg and vanilla until smooth. Add remaining dry ingredients, mixing until combined. Finally, add sour cream, mixing until all ingredients are incorporated and no streaks remain. Scrape bowl as needed.
Divide batter between two bowls, roughly 6 large scoops per bowl.
Scatter topping ingredients into the bottom of the bundt pan. Carefully spread out half of the batter over streusel topping. Sprinkle with brown sugar filling and top off with remaining batter.
Bake for 50-55 minutes or until dark and a toothpick inserted comes out clean.
Invert onto a wire rack to cool completely.
In a small bowl, whisk together powdered sugar and milk. Drizzle over cooled cake. Slice with a sharp knife and serve. Store leftovers in an airtight container.
*This recipe is written for an 8-inch (6 cup) bundt pan. Measure your pan before beginning. Double ingredients for a 10-inch (12 cup) bundt pan.An original recipe from Baked by Rachel