SallysBakingAddition.com credits Cooks Illustrated for the inspirationGrease and flour a 9x9x2 inch baking dish, or 9 inch springform pan. I (mdy) doubled the recipe and used a large sheet pan as I wanted the cake to be thin for coffee hour at church.

2 cups flour
1 cup + 2T sugar
1t salt
10 T butter (1.25 stick), lightly softened
1t baking powder
1/2 t baking soda
3/4 buttermilk (you can make your own by adding 1T lemon to 1c milk and let it sit for 5-10 minutes)
1 large egg, room temperature
2t vanilla
2/3 c brown sugar
2t cinnamon

Preheat oven to 350
Whisk flour, sugar and salt in a large mixing bowl. Cut in butter until mixture resembles course crumbs. Set aside 1 cup of this mixture for the topping. Add baking powder and baking soda to remaining flour.

In the mixer, mix buttermilk, egg and vanilla. Gradually add dry ingredients until you have a thick batter. Vigorously mis until it is smooth and fluffy, like frosting, about 2 minutes+. Spoon into your prepared pan.

Add brown sugar and cinnamon to the reserved flour mixture. Sprinkle over the batter, pressing lightly so it sticks. Bake until center is firm and a toothpick inserted comes out clean. Thick cake will take about 55 minutes. My flatter cake took 40. Cool before slicing.