Heat oven to 350° F
Grease skillet with butter
In a medium bowl, whisk together flour, baking powder, and baking soda. Set aside.
With a stand or electric mixer, beat together butter and brown sugar until light and fluffy.
Add eggs one at a time to butter mixture. Beat well after each egg.
Beat in vanilla.
With the mixer on low, beat in half the flour mixture until thoroughly combined.
Add in yogurt and mix until thoroughly combined.
Add in remaining flour and mix until thoroughly combined.
This is a stiff batter. Spread into buttered skillet. Set aside and prepare topping.
In a small bowl, combine brown sugar, flour, and cinnamon.
Add firm butter and cut with pastry cutter or work into small crumbs with your fingers.
Sprinkle topping evenly over cake batter.
Bake for 35 -40 minutes. Check with a cake tester or toothpick at 30-35 minutes.
Cut and serve warm or cool.
Store leftovers in a covered container.