Melt the unsalted butter in a saucepan. Use a silicone pastry brush to grease the bottom and sides of a 9x13-inch cake pan with some of the melted butter. Place the pan in the refrigerator to set the butter.

In a large bowl, sift together the brown sugar, cinnamon, flour, and kosher salt. Add the melted butter and stir until large clumps form. If needed, add more melted butter in tablespoon increments. Aim for a variety of crumb sizes, including some 'cherry tomato-size' crumbs, to spread over the cake.

Preheat oven to 350°F. Remove the prepared cake pan from the refrigerator. Whisk together the flour, baking powder, baking soda, and salt.

In a medium bowl, add the softened butter and whip until light. Add the granulated sugar and beat until light and fluffy. Add both eggs one at a time, blending after each addition, then add the buttermilk and vanilla. Blend until just incorporated; it will be thick but fluid.

Add the flour mixture to the buttermilk mixture in two separate additions, mixing gently between each addition. The cake batter is now complete.

Transfer the cake batter to the prepared baking pan, ensuring you scrape all the batter from the bowl. Spread the batter into the corners using the back of a wooden spoon or an offset spatula, smoothing the top.

Evenly distribute the cinnamon crumb topping over the batter, placing the largest lumps in each corner and a few on the sides and middle. Work from the edges to the center to prevent crumbs from sinking.

Bake until a toothpick inserted in the center comes out clean, and the streusel topping is golden-brown and slightly crunchy, anywhere from 50 to 70 minutes. Your oven may bake differently than mine, so begin testing for doneness at 50 minutes and re-test every 10 minutes if additional bake time is required.

Cool the cake in the pan on a wire rack, then cut into 15 squares. Dust with powdered sugar once completely cooled and serve.