Heat the oven to 350°F (175°C). Grease a 9 x 13-inch baking pan and line it with parchment, leaving enough overhang to lift the cooled cake out easily.

Make the crumb topping first by whisking together the flour, packed light brown sugar, cinnamon, and salt. Stir in the melted butter until large, irregular crumbs form, then set the mixture aside or refrigerate it for 10–15 minutes for especially distinct crumbs.

For the cake, whisk together the flour, baking powder, baking soda, and salt in a separate bowl. In a large mixing bowl, beat the softened butter and granulated sugar for about 2–3 minutes, until light and fluffy.

Beat in the eggs one at a time, mixing after each addition. Add the vanilla extract and mix briefly until incorporated.

Add the dry ingredients in two or three additions, alternating with the sour cream. Mix only until combined; the batter will be fairly thick.

Spread the batter evenly in the prepared pan. Scatter the crumb mixture over the entire surface and press it very lightly so the topping adheres without crushing the crumbs.

Bake for 35–45 minutes, checking at the lower end of the range. The center should be set, and a toothpick inserted into the cake portion should come out with a few moist crumbs.

Cool the cake in the pan for 20–30 minutes before slicing. If desired, sift 1/2 to 1 cup of confectioners’ sugar over the cooled cake just before serving.