Step 1

Process gingersnaps in a food processor until fine crumbs form. Add flour, brown sugar, cinnamon, and salt and pulse until combined. Transfer flour mixture to a large bowl and gradually drizzle in melted butter, mixing until large sandy clumps form and no dry spots remain. Add ginger and mix just to combine. Set crumb topping aside until ready to use. Wipe out food processor bowl (no need to wash it; you’ll use it for the cake).

Step 2

Preheat oven to 350°. Lightly grease a 13x9" pan with butter, then line with parchment, leaving an overhang on both long sides; butter parchment. Pulse flour, granulated sugar, baking powder, salt, and baking soda in food processor until combined. Add 10 Tbsp. butter and pulse until mixture is sandy. Add eggs, sour cream, and vanilla and pulse continuously until mixture is very smooth and homogenous, about 2 minutes. Scrape down sides and pulse once more.

Step 3

Spread batter in prepared pan. Scatter crumb topping over, squeezing small handfuls with your palms to create some larger crumbs and slightly favoring the outside 2" around the perimeter.

Step 4

Bake cake until crumb is golden brown and a tester inserted into the center comes out clean, 40–50 minutes. Let cool.

Step 5

Run a thin knife around edge of pan to help release cake. Lift parchment up and transfer cake to a cutting board or platter; discard parchment. Cut cake into squares.