Preheat your oven to 375°F (190°C) and grease and flour a 9-inch round baking pan, or line it with parchment paper for easy removal.

In a medium bowl, combine the flour, brown sugar, and cinnamon for the streusel topping. Add the cold cubed butter and use your fingers or a pastry blender to mix until coarse crumbs form, then set aside.

In a large bowl, whisk together the dry cake ingredients: flour, granulated sugar, baking powder, and salt. In a separate bowl, combine the wet ingredients: egg, milk, melted butter, and vanilla extract. Gradually add the wet ingredients to the dry ingredients, mixing until just combined and being careful not to overmix.

Pour half of the cake batter into the prepared pan and sprinkle half of the streusel topping over it. Carefully spread the remaining batter over the streusel layer, then top with the rest of the streusel. Bake for 35-45 minutes, or until a wooden skewer inserted into the center comes out clean.

Allow the coffee cake to cool in the pan for 10 minutes before inverting it onto a wire rack to cool completely. If desired, whisk together powdered sugar, milk, and vanilla for the glaze, then drizzle it over the cooled cake.