Make the Cake

1. Sift together the flour, baking powder, baking soda and salt in a medium bowl.
2. In the bowl of a standing mixer fitted with the paddle attachment, cream the butter on medium-low speed for 2-3 minutes. Add the sugars and mix on medium-low for 1-2 minutes, until the batter is fluffy and whipped. Add the eggs and vanilla and mix on medium-low until just combined, scraping down the sides and bottom of the bowl. Detach the bowl from the mixer.
3. Use a spatula to fold half of the flour mixture into the egg mixture. Fold in 1/2 cup of the kefir, then repeat with the remaining flour mixture and kefir until everything is just combined. (Do not overwork the batter.)
4. Cover the bowl with plastic wrap and store in the refrigerator all night, or up to 8 hours.
5. When you are ready to bake, use the butter wrappers to butter and flour a 9×13 baking dish. Preheat the oven to 350F.
6. Remove the batter from the refrigerator and spread the batter into the prepared pan. Set aside.

Make the Topping

1. Mix together the flour, granulated sugar and salt in a medium bowl. Cut the butter into the flour mixture with your fingers or a pastry blender until the butter is completely incorporated into the flour in small chunks. The topping can be made at night and stored in the refrigerator with the cake batter.
2. Sprinkle half of the mixture on top of the cake.
3. In a small bowl, combine the cinnamon and cocoa powder and sprinkle evenly over the topping.
4. Sprinkle the remaining topping over the cinnamon and cocoa. Bake for 42 minutes until the middle springs back when pressed with your finger.
5. Let the cake sit in the pan for 1 hour until completely cooled. Dust with powdered sugar and serve.