Preheat the oven to 350 F (177 C). Lightly grease and line the bottom and long sides of a 9 x 9 inch (23 cm) cake pan with parchment paper.
In a large bowl, whisk together the flour, brown sugar, granulated sugar and ground cinnamon for the streusel topping. Work in the cold butter cubes until a sand like texture develops. There will be a few bigger crumbs, but most should be finer, smaller pieces. Place the bowl in the fridge until the coffee cake is ready for topping.
¾ cup (90 g) all purpose flour, ¼ cup (50 g) light brown sugar, ¼ cup (50 g) granulated sugar, 1 teaspoon ground cinnamon , 6 tablespoons (87 g) unsalted butter, cold and cubed
In a medium bowl whisk the all purpose flour, baking powder, baking soda and salt together until well combined.
2 cups (240 g) all purpose flour, 2 teaspoons baking powder, ¼ teaspoon baking soda , ½ teaspoon salt
In a large mixing bowl, beat butter and sugar together until light and fluffy. This takes about 2-3 minutes on medium speed. On low, beat in the eggs and vanilla extract until smooth and well combined.
½ cup (113 g) unsalted butter, room temperature, 1 cup (200 g) granulated sugar, 2 large eggs, room temperature, 1 teaspoon vanilla extract
Alternating dry ingredients and buttermilk, mix in ⅓ of the dry ingredients into the wet, followed by ½ of the buttermilk. Repeat until all the ingredients are just combined.
1 cup (237 ml) buttermilk, room temperature
Evenly spread half coffee cake batter into the prepared pan. Use ⅓ to ½ of the streusel to top the first layer of cake. Repeat with a second layer of coffee cake and finish with the remaining cinnamon streusel.
Bake the coffee cake for 35 - 45 minutes or until a toothpick inserted into the center of the coffee cake comes out clean or with a few moist crumbs. Cool the pan in the pan for 15-20 minutes. Gently lift the coffee cake out of the pan using the parchment paper. Transfer to a serving plate or serve right from the pan.
Make the vanilla glaze by whisking powdered sugar with vanilla extract and milk or heavy cream. Top the crumb cake coffee cake with the glaze, slice and enjoy!
1 - 1½ cups (120 - 180 g) powdered sugar, 2 - 3 tablespoons (30 - 45 ml) milk or heavy cream, 1 teaspoon vanilla extract or vanilla bean paste