Preheat the oven to 350°F. Line a 9x13-inch baking pan with parchment paper or lightly flour and grease with non-stick cooking spray.
Prepare the cinnamon layer. In a bowl, mix together the granulated sugar, brown sugar, and cinnamon. Set aside.
Prepare the cake batter. In a medium-sized bowl, whisk together the dry ingredients - flour, baking powder, cinnamon, and salt. Set aside.
In a large mixing bowl, using an electric mixer, beat the softened butter and cream cheese until creamy. Beat in both sugars until smooth. Scrape the bottom and the sides of the bowl to make sure all ingredients are all well incorporated. Add 3 eggs, one at a time, beating after each addition until well combined. Stir in vanilla extract using a rubber spatula.
Gradually stir in the dry ingredients and buttermilk until smooth and no lumps of flour remain.
Pour half of the batter into the prepared baking pan. Spread evenly. Top with the cinnamon mixture and spoon the remaining cake batter on top. Using a knife or a spatula, carefully spread the batter evenly on top. Set aside.
Prepare the streusel topping. In a bowl, mix together flour, brown sugar, cinnamon, and salt using a fork. Add the softened butter and mix it into the dry ingredients until combined and crumbly. Sprinkle over the batter.
Bake in the preheated oven for 45-55 minutes or until a toothpick inserted in the center of the coffee cake comes out clean. Remove from the oven and let cool completely on a cooling rack.
While the coffee cake is cooling, prepare the glaze. In a small bowl, whisk together powdered sugar and milk until combined. If the glaze is too thin, add more powdered sugar, if too thick - add more milk.
Divide the cooled coffee cake into 16 squares and drizzle with glaze before serving. Enjoy!