Preheat oven to 350 degrees or 350F.

Grease a 9 × 12 × 2 inch cake pan or 9×13 baking dish. You can also butter flour the baking dish using leftover butter wrappers.

In a medium bowl, sift together flour, baking powder, baking soda, and salt. This hand sifting helps remove lumps.

In a large bowl or stand mixer bowl, cream butter on medium-low speed for 2-3 minutes. Add the sugars and mix 1-2 minutes until the batter is fluffy and whipped.

Add eggs, vanilla extract, and kefir. Mix until combined mixture forms.

Fold the flour mixture into the egg mixture. Add ½ cup kefir, then repeat with remaining flour mixture until everything is just combined. Do not overmix. Small lumps are fine.

In a small bowl, mix flour, granulated sugar, salt, and cinnamon. Cut butter into the flour mixture with fingers or a pastry blender until small chunks form.

Pour the batter into the greased pan and spread evenly. Sprinkle the crumb topping evenly across the surface.

Bake for 40–42 minutes until a knife inserted into the middle cake comes out dry and the cake springs back when pressed.

Let the cake cool on a rack for 1 hour. Dust with powdered sugar before serving.