Preheat oven to 350 degrees.  Spray a 9 inch springform pan with butter spray and sprinkle with 1 tablespoon of flour.

In large bowl of a stand mixer, mix together flour, sugar, and salt until combined.  Using a pastry blender, cut butter into mixture, until butter/flour mixture looks like coarse crumbs.  Scoop out and set aside 1 cup of the butter/flour mixture.

Mix the baking powder and baking soda into remaining butter/flour mixture.  Add in the room temperature buttermilk, egg, and vanilla.  Mix well.  Batter will be very thick.  Mix on high until batter is fluffy, and looks similar to whipped frosting consistency (about 2 minutes).

Spoon batter into springform pan and smooth top.

Add the brown sugar, cinnamon, and chopped walnuts (if using) to the reserved 1 cup of butter/flour mixture and toss together with fork. Sprinkle butter/flour/brown sugar/cinnamon mixture on top of  batter and spread evenly over batter.  Press lightly into batter until it sticks.

Bake for 50-55 minutes, or until toothpick inserted into center comes out clean.  50 minutes was perfect for our cake.

Cool in pan for 10 minutes, remove side of  springform pan.  Let cake cool for about an hour.  Slice and Enjoy!!