Melt two sticks of unsalted butter in a saucepan. Use a pastry brush to grease the bottom and sides of a non-stick 13x9 cake pan with just a bit of butter. Place the pan into the refrigerator while preparing the remaining parts of the cake so the butter sets.

Mix brown sugar, cinnamon, and 1 teaspoon Kosher salt in a small bowl, stirring well. Add to the barrel of a food processor. Add melted butter. Pulse until mixed, about 6 to 8 pulses. Add 3 ½ cups of the flour and pulse until large clumps form, about 4 to 6 pulses. Spill the cinnamon sugar clumps out into a bowl and use your hands to squeeze bits of the mixture together. The mixture will clump easily, but add additional melted butter, 1 tablespoon at a time if necessary. Aim for a variety of sizes of crumbs.

Preheat oven to 350°F. Remove the prepared cake pan from the refrigerator. If using anything other than non-stick, dust lightly with flour, tapping out as much excess as possible. Combine the remaining 2 cups flour, baking powder, baking soda, and ½ teaspoon salt in a mixing bowl. Whisk to combine.

Add the remaining 1 ½ sticks of softened butter and whip until it's light in the bowl of a stand mixer fitted with the paddle attachment. Add in granulated sugar and beat until light and fluffy.

Add the eggs, one at a time, blending after each addition. Add buttermilk and vanilla extract and mix until just incorporated, followed by flour mixture in two separate additions. Scrape the sides of the bowl down. Again, beat just until incorporated but no more.

Transfer cake batter to prepared baking pan spreading it into the corners using an offset spatula if necessary to push the mixture. Smooth the top.

Evenly distribute the crumb topping so each cut square has a few lumps. Add large lumps of the streusel topping in each corner and a couple on the sides and middle. Then, working from the edges in, follow with the remaining crumbs. Working your way to the center prevents the crumbs from sinking.

Bake in a preheated oven for 50 minutes to 1 hour or until the insertion and removal of a toothpick comes out clean. The streusel topping will be golden brown and slightly crunchy in spots.

Cool the cake in the pan atop a wire cooling rack for 1 hour, longer if you prefer to eat it completely cooled. Cut into thirds lengthwise and into fifths from side to side to yield 15 'squares.' Use a fine mesh sieve to dust with Confectioners' sugar once cut, then use a spatula to remove individual pieces from the cake pan.

Plate individual squares and dust with additional powdered sugar to serve.