Make the Cake

1. Place a fine mesh strainer over a medium bowl and measure the flour, baking powder, baking soda, and salt. You may need to do this in batches, depending on the size of your strainer. Sift the ingredients into the 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. Pour in the sugars and mix on medium-low for 1-2 minutes, until the batter is fluffy and whipped. Then 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 ½ cup of the kefir, then repeat with the remaining flour mixture and kefir until everything is combined. (Make sure not to overwork the batter.)
4. Cover the bowl with plastic wrap and place 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. Then set it aside.

Make the Topping

1. Combine 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 placed in the refrigerator with the cake batter.
2. Sprinkle half of the mixture on top of the cake.
3. In a small bowl, mix the cinnamon and cocoa powder and sprinkle evenly over the topping.
4. Sprinkle the remaining topping over the cinnamon and cocoa powder. Then bake for 42 minutes until the middle bounces back when pressed with your finger.
5. Let the cake sit in the pan for 1 hour until completely cooled. Dust it with powdered sugar and serve.