1. Butter an 8x4" loaf pan, then line it with parchment paper to form a sort of hammock, where there are two sides with parchment paper sticking up. This will make it easy to pull the loaf out of the pan when it's done baking.
2. Preheat the oven to 350°F / 175°C.

Cinnamon Streusel

1. In a medium sized bowl, melt the butter. Add in the flour, sugar, and cinnamon. Use a fork to mix the ingredients together until you have formed little clumps of dough. Set aside.

Coffee Cake

1. In a small bowl, whisk together the flour, baking powder, and salt. Set aside.
2. In a medium sized mixing bowl, with a hand mixer (or stand mixer fitted with the paddle attachment) beat together the softened butter and sugar for 3-4 minutes, until the mixture is light and fluffy.
3. Scrape down the sides of the bowl, then beat in the egg and vanilla until well combined.
4. Pour half of the flour mixture into the bowl, then use a rubber spatula to gently fold the ingredients together just until no flour streaks remain. Add in all of the sour cream and milk, fold, then gently fold in the rest of the dry ingredients.
5. Add half of the cake batter to the prepared loaf pan, and top it with ~1/3 of the streusel. Top the streusel with the remaining batter, then top that with the remaining streusel.
6. Bake for 35-45 minutes, until a toothpick inserted in the center of the coffee cake comes out with just a few moist crumbs.

Glaze

1. In a small bowl, whisk together the powdered sugar and milk. If you'd like a thicker glaze, add more powdered sugar. If you'd like a thinner glaze, add more milk.
2. Once the cake has cooled to room temperature, top it off with the glaze, serve, and enjoy!