"This is a recipe I have tweaked over the years. High quality cinnamon is a must. Old or weak cinnamon will bring the taste down quite a bit. I have played with the streusel many times, but have always come back to this one. Chopping the oatmeal finely disguises it a bit while still giving the topping some density."
Preheat oven to 350. Grease bottom and sides of a 9” round cake pan.
In a large bowl, cut the butter into 1 Tbsp. pieces and set aside to soften.
In a medium bowl, add the flour, cinnamon, baking powder, and salt. Blend well and set aside.
In another medium bowl, prepare the streusel: Chop the quick oatmeal (in a food processor, with a knife, or mortar & pestle), until very fine and add to the bowl. Add the brown sugar, flour, and cinnamon. Blend well. Add the melted butter, and blend well. It should become like a thick dough. Set aside.
With a wooden spoon, cream the softened butter in the large bowl until smooth. Add the 1 cup of sugar and cream well. Add the egg. Mix until blended, then beat until smooth. Add half of the flour mixture and half of the 1/2 cup milk. Blend well. Add the rest of the flour mixture and the rest of the milk. Blend well, then beat the mixture until it is smooth with no lumps.
Pour the batter into the prepared pan. Smooth to the edges with a spatula. Add the streusel by rubbing between your fingers and distributing it as evenly as possible, all the way to the edges of the pan.
Bake for 35-45 minutes, or until a toothpick inserted in the center comes out clean.
Let cool for 10-15 minutes, then run a knife around the edge of the pan to loosen and remove from the pan to a plate. (It can be flipped over to remove. The streusel bakes in and will not fall off.) Cut as desired. Serve.