Heat oven: To 350°F (175°C). Lightly coat an 8-inch square or 9-inch cake pan with butter or nonstick spray. For extra security, line it with parchment paper.
Make crumbs: Whisk butter, sugars, cinnamon, and salt together until evenly mixed. Add flour and mix until it disappears. It’s going to be very thick; press it evenly into the bottom of the bowl and set it aside.
Make cake: Beat butter with sugar until lightened and fluffy. Add egg, water or milk, sour cream, and vanilla and beat until combined. Sprinkle surface of batter with baking powder and salt, and beat well to combine. Add flour and mix only until it disappears.
Assemble: Scrape batter into prepared cake pan and smooth it flat. Use a spoon to loosen the crumbs from the bottom of the bowl where they’ve rested, breaking them up into big and small rubble. Sprinkle evenly over the batter.
Bake: Bake for about 30 to 35 minutes, or until a toothpick inserted into the cake just below the crumbs comes out batter-free. Check in a few spots, in case it has baked unevenly.
Cool to lukewarm, if you can bear it, before dusting it generously with powdered sugar and cutting the cake into squares or wedges.
Do ahead: This cake keeps in an airtight container at room temperature for 3 to 4 days, but I like it best in the first two.