Heat oven to 375 degrees F.Optional: Line the baking dish with parchment paper.Grease a 9 by 9-inch baking dish with 2-inch sides.
In a large bowl, combine all the dry ingredients.
In a medium bowl, combine the butter, egg, and buttermilk.
Add wet ingredients to the dry ingredients and still until combined.
Pour half of the cake batter into a prepared dish.
Streusel Filling
Combine softened butter, cinnamon, flour, brown sugar, and (optional) chopped nuts in a small bowl with a fork.
Sprinkle the cinnamon streusel over the cake batter.
Next, spread the remaining cake batter over the streusel.
Crumb Topping
Cut cold butter into small cubes then add to a small bowl.
Add the remaining ingredients and combine with a fork or pastry blender.
Sprinkle the crumb topping over the batter.
Bake for 30 to 40 minutes or until an inserted toothpick comes out clean.Cool for 15 minutes on a cooling rack.Then, lift the cake out of the pan and cool completely.
Drizzle Topping
In a small bowl, combine powdered sugar with 2 - 3 teaspoons of milk with a spoon. Add more milk if necessary for a smooth drizzle glaze.
Add the drizzle glaze over the cooled cake with a spoon.
Notes
Easy classic coffee cake is a moist, buttery cake with a ribbon of sweet cinnamon streusel in the center and topped with a crumb streusel. It is the official breakfast cake! It pairs perfectly with a cup of coffee for an early morning meal or a pick-me-up any time.