• Preheat oven to 350°F. Line an 8 by 8 inch baking dish (or nonstick frying pan) with parchment paper.

Crumbs

• In a mixing bowl whisk together brown sugar, granulated sugar, cinnamon + salt. Pour in melted butter and stir to combine. Slowly add flour and mix with spoon until no streaks of flour remain. Set aside to rest while cake batter is prepared.

Cake

• In a mixing bowl whisk together flour, baking powder, baking soda + salt for 20 seconds, set aside. In a second mixing bowl whisk together butter + granulated sugar until pale + fluffy. Mix in egg + vanilla. In liquid measuring cup used to measure milk, whisk together sour cream (or yogurt) with milk. Add in 1/3 of the flour mixture to the butter mixture and blend just until combined, then add 1/2 of the milk mixture and mix just until combined. Repeat process once more, then end by mixing in last 1/3 of the flour mixture until nearly combined. Scape down sides and bottom of bowl using a rubber spatula and fold batter to fully incorporate.
• Pour batter into prepared baking dish and spread into an even layer. Break crumb mixture into pea sized crumbs and sprinkle evenly over batter. Bake in preheated oven until toothpick inserted into center comes out free of batter, about 32 – 35 minutes. Cool in pan 10 minutes, then using parchment overhang lift cake from pan and cool on a wire rack. Store in an airtight container.