Heat oven to 350° F and grease skillet with some butter.
In a medium bowl, whisk together flour, baking powder, and baking soda. Set this flour mixture to the side while you mix the wet ingredients.
2 1/2 cups all purpose flour, 2 teaspoons baking powder, 1/2 teaspoons baking soda
In the bowl of a stand mixer with paddle attachment (or in a large bowl with a hand mixer) cream butter and brown sugar until light and fluffy.
1 cup brown sugar, 1/2 cup butter
Add eggs one at a time to butter mixture on medium speed. Beat well after each egg.
2 eggs
Add vanilla extract.
1 teaspoon vanilla extract
With the mixer on low speed, beat in half the dry ingredients until thoroughly combined. Take a rubber spatula and scrap down the edges.
Add yogurt and mix until well combined.
3/4 cup plain Greek yogurt
Add remaining flour and mix until thoroughly combined. This batter will be thicker than traditional cake batter.
2 1/2 cups all purpose flour
Pour batter into buttered skillet. Set aside and prepare topping.
In a small separate bowl, combine brown sugar, flour, and cinnamon.
3/4 cup brown sugar, 1/2 cup flour, 2 teaspoons cinnamon
Add firm butter and cut with pastry cutter or work into small crumbs with your fingers.
6 tablespoons butter
Sprinkle the streusel topping mixture in an even layer over the top of the batter.
Bake for 35 -40 minutes. Check with the center of the cake a cake tester or toothpick at 30-35 minutes. Cut and serve warm or cool.