In a large mixing bowl, whisk together the flour, baking powder, salt, and cinnamon. Set aside.
In a 2-cup liquid measuring cup, whisk together the yogurt and whole milk. Set aside.
In the bowl of a stand mixer fitted with the paddle attachment, cream the butter and brown sugar until light. Beat in the egg, mixing until incorporated. Stir in the vanilla extract.
Alternate adding the flour and yogurt mixtures, mixing until just incorporated.
Spread in the prepared baking dish and sprinkle the topping over the batter.
Bake for 24-30 minutes, or until a cake tester inserted in the middle comes out clean.
Cool completely before removing from pan and serving.