Place the rack in the center of the oven. Preheat the oven to 350ºF. Spray a 9×13-inch glass baking pan with cooking spray, then dust with flour. Tap out the excess.
In a mixer with the paddle attachment, beat the butter and sugar until light and fluffy. Add in the egg and sour cream and mix until combined.
In a small bowl or measuring cup, combine the milk and the vanilla. In another bowl, whisk together the 2 cups flour and the baking powder. Add in the salt.
Turn the mixer on low speed and alternately add the wet ingredients and the dry ingredients, starting and finishing with the dry ingredients, stopping to scrape down the sides of the bowl several times.
Spread the batter evenly into the prepared baking dish.
In another bowl, whisk together the flour, brown sugar and cinnamon until completely combined. Add in the melted butter and stir until them mixture is completely moistened. Use your hands to roll the mixture between your fingertips until small balls form. Scatter the topping over the top of the batter in the pan, right from your hands as you roll it.
Bake for about 30 minutes, or until the center is firm to the touch and it springs back lightly when pressed. If the cake is not done after 30 minutes, reduce the oven to 325ºF and continue to bake in 5 minute increments until done.
Remove the cake from the oven and lightly dust with powdered sugar. Allow the cake to cool completely. Once the cake is cool, dust generously with powdered sugar, slice and serve.