Streusel Topping

1. In a medium bowl, combine the flour, sugars, cinnamon, salt and melted butter. Use a fork to mix the crumble, making sure all of the dry ingredients are coated in the melted butter.
2. Place uncovered in the refrigerator until ready to use. 

Cake

1. Preheat the oven to 350°F. Line an 8 x 8 inch pan with parchment paper for easy removal of the cake once baked. Set aside.
2. In a medium bowl, whisk together the flour, baking powder and salt. Set aside.
3. Using a hand mixer (or a stand mixer), cream together the butter and oil until light and creamy, about 1 minute. 
4. Slowly add the granulated sugar and mix on medium speed for about 2 minutes until light and fluffy. Scrape down the sides of the bowl.
5. Add the eggs, one at a time, followed by the vanilla. Mix to combine well.
6. Add the dry ingredients to the butter and egg mixture, alternating with the sour cream. Begin and end with the dry ingredients. Scrape down the sides of the bowl.
7. Transfer the batter to the prepared pan, pressing it evenly into the corners of the top and smoothing the top with a spatula.
8. Remove the streusel topping from the refrigerator and use your hands to distribute it evenly on top of the batter. Do not press it into the batter.
9. Bake in the center rack of the oven for 45 to 50 minutes or until the top is slightly golden brown and a toothpick inserted into the center of the cake comes out clean or with a few crumbs remaining. If the streusel is browning too quickly, loosely cover the top of the cake with a piece of aluminum foil for the last 10 to 15 minutes of baking. 
10. Remove the cake from the oven and cool completely before adding the glaze. Alternatively, dust the top with powdered sugar just before serving. 

Glaze

1. To make the glaze, whisk together the powdered sugar, vanilla and milk until smooth but pourable. If the glaze is too thin, add more powdered sugar. If too thick, add more milk.
2. Once desired consistency is reached, use a spoon or whisk to drizzle over the top of the cake.