In a small bowl whisk together the brown sugar, flour, and cinnamon. Set aside.

In a medium bowl, whisk together the flour, brown sugar, cinnamon, and salt. Add the diced butter. Use your clean fingers or the tines of a fork to press and crumble the butter into the dry mixture until the mixture is crumbly and resembles wet sand. For larger crumbles, squeeze a handful of the mixture together and break apart into large pieces. Set aside.

Preheat the oven to 350°F. Lightly grease an 8x8-inch square baking pan and line with parchment paper.

In a medium bowl, whisk together the flour, baking powder, and salt.

In a large mixing bowl or the bowl of a stand mixer fitted with the paddle attachment, beat the butter and both sugars on medium speed until light and fluffy, about 3 minutes. Add the eggs one at a time, beating well between each addition. Scrape the bowl down, then add the sour cream and vanilla and mix on low speed until combined. Scrape down the bowl.

Add the dry mixture to the wet mixture and beat on low speed until just combined, then use a spatula to scrape the bowl down and mix any stray bits in.

Spread half of the cake batter into the prepared pan and smooth it into an even layer. Sprinkle the cinnamon filling on top. Dollop the remaining batter over the cinnamon filling, then spread it out into an even layer by moving from the center to the edges. Sprinkle the streusel over the top, starting at the edges, and working in towards the center. (This helps keep make sure there’s enough topping on the edges to keep the cake from rising unevenly.

Bake for 55 minutes or until a toothpick inserted in the center comes out clean. Allow the cake to cool in the pan for at least 30 minutes before serving.

If making the glaze, whisk together the powdered sugar, milk, and vanilla in a small bowl until smooth. Drizzle over the coffee cake before or after slicing.