Streusel
Add the flour, cinnamon, and sugar to a bowl and mix.
Next, add the cubed butter and cut in with a pastry cutter until the mixture resembles the shape of peas.
Coffee Cake
Add the flour, sugar, baking powder, and salt to a medium bowl and mix.
Cut in the butter with a pastry cutter until it resembles the shape of peas.
Add the buttermilk, vanilla extract, egg, and milk and mix until combined.
Pour the mixture into the prepared baking dish and sprinkle the streusel mixture evenly over the top.
Bake for 25 to 30 minutes or until a cake tester or toothpick comes out clean. The top will be melted, crispy, and golden brown.