Preheat your oven to 350°F (175°C). Grease a 9x13-inch baking pan with butter or nonstick spray.

In a large bowl, combine the cake mix, eggs, oil, and sour cream. Stir until just combined — do not overmix. A few small lumps are fine. The batter will be thick.

Pour the batter into the prepared pan and spread it evenly with a rubber spatula.

In a separate small bowl, make the crumble: mix the sugar, flour, melted butter, cinnamon, and salt with a fork until it forms coarse, pea-sized crumbs. If it’s too wet, add a tablespoon more flour; if too dry, add a teaspoon more melted butter.

Sprinkle the crumble evenly over the batter. Press it down lightly with your fingers so it adheres during baking.

Bake for 30–35 minutes, until a toothpick inserted in the center comes out clean or with a few moist crumbs. The top should be golden brown and the edges pulling away from the pan.

Let the cake cool in the pan on a rack for at least 20 minutes before cutting. Serve warm or at room temperature.