Step 1

In a large bowl, whisk together the melted butter, granulated sugar, brown sugar, cinnamon, and salt. Using a spoon, stir in the cake flour one cup at a time, until the mixture forms a cohesive dough, like a very thick and clumpy cookie dough. As you add the last cup of flour, the mixture will be difficult to stir, so feel free to ditch the spoon and use your hands to mix it in. Set aside.

Step 2

Preheat the oven to 350°F and set an oven rack to the middle position. Grease a 9 x 13-in cake pan.

Step 3

In a medium bowl, whisk together the flour, baking soda, baking powder, and salt. Set aside.

Step 4

In the bowl of an electric mixer, beat the softened butter and granulated sugar on medium-high speed, scraping the bowl as necessary, until light and fluffy, 2 to 3 minutes. Add the eggs one at a time, beating until incorporated after each addition (don't worry if the batter looks a little curdled at this point). Beat in the vanilla and scrape the bowl to make sure everything is evenly combined. Add the flour mixture alternately with the sour cream (half of the flour/all of the sour cream/then the remaining half of the flour), mixing on low speed with each addition, until just combined.

Step 5

Spread the cake batter evenly in the cake pan (it will only come about ¾ inch up the sides of the pan). Top the batter with the streusel crumbs, using your fingers to break it apart and form it into roughly ½-inch chunks. Bake for 40 to 45 minutes, until a cake tester inserted into the middle of the cake comes out clean. Let the cake cool fully on a rack. Dust with powdered sugar and cut into squares to serve. Cover the cake with aluminum foil and store at room temperature; it will keep nicely for up to 3 days. To keep the cake fresher longer, store it in the refrigerator but bring it to room temperature before serving.