1. Preheat oven to 350°F. Grease or line an 8×8-inch pan with parchment paper. Allow the butter, eggs, and sour cream to come to room temperature then prepare the streusel and apples.
2. Prepare the streusel topping by combining the butter with the dry ingredients using a spoon until well combined with a mixture of crumbles and loose crumbs. Set aside.
3. Prepare the apples. Peel and dice apples about the size of a dime. Toss with the dry ingredients: brown sugar, cinnamon, and apple pie spice. Set aside.
4. In a stand mixer, cream the butter and sugar together for several minutes until light and fluffy. Next, add the egg, beat until well incorporated, and scrape down the sides of the bowl as needed. Then add sour cream and vanilla extract and beat until well combined.
5. In a separate bowl, combine the remaining dry ingredients and slowly add to the batter, beating until incorporated. Then use a spatula and mix by hand until well combined. 
6. Pour about half of the batter into the bottom of the pan and spread evenly. Then sprinkle half of the streusel over the batter. Top with the remaining batter, then add the apple flavor. Spoon the apples over top, leaving any remaining juices in the bowl. Sprinkle with remaining streusel.
7. Bake at 350°F for 45-50 minutes. Check the cake for doneness by inserting a toothpick into the center of the cake. If the toothpick comes out clean, then the cake is done. Allow to cool completely or serve slightly warm. 

For the glaze

1. Brown the butter in a saucepan over medium-low heat. Slowly bring the butter to a boil, stirring occasionally. Once boiling, the bubbles will go from big to small, and once the milk solids start to turn golden brown, stir constantly until the golden color starts to darken. Immediately remove from the heat and pour into a heatproof bowl. Cool for several minutes.
2. Combine the butter with the powdered sugar, then add the milk 1 tablespoon at a time until you reach the desired consistency. Drizzle over the finished coffee cake before serving.