Preheat oven to 350 degrees. Spray 9″ glass baking dish with cooking spray.
In a small bowl- mix together flour, baking powder, salt & cinnamon. Set aside.
In a mixer bowl- beat your butter & sugar together.
Beat in egg.
Add in the dry mixture- slowly.
As it’s mixing- slowly pour in the milk.
Combine all streusel ingredients in a small bowl & whisk until combined.
Pour 50% of the cake batter in pan.
Drizzle in 50% of the streusel mixture- swirl with a knife a little bit.
Top with remaining cake batter & then with the remaining streusel mix.
Bake 35-40 minutes. Remove & allow to cool completely before adding icing.