Step 1

Begin by melting two sticks of unsalted butter in a saucepan. Use a silicone pastry brush to grease the bottom and sides of the cake pan with a bit of the butter as it melts. Place the prepared pan into the refrigerator while preparing the remaining parts of the cake.

Step 2

In a small bowl, combine brown sugar, cinnamon, and Kosher salt. Stir well breaking up all lumps. Scrape into the barrel of a food processor. Add melted butter. Pulse until mixed, about 3 to 5 pulses. Add the flour and pulse until large clumps form, about 5 to 8 pulses. The flour should just be moistened and have no visible dryness. Spill the cinnamon sugar clumps out into a bowl and use your hands to squeeze some of the mixture together. The mixture should immediately hold together. If yours does not, add additional melted butter in 1-tablespoon increments.

Step 3

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

Step 4

In the bowl of a stand mixer fitted with the paddle attachment, add softened unsalted butter and whip until it's light. Add in granulated sugar and beat until light and fluffy.

Step 5

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

Step 6

Transfer cake batter to prepared baking pan spreading it into the corners and from side to side, end to end. Smooth the top with a rubber spatula or the back of a wooden spoon.

Step 7

The streusel topping is considered the pinnacle of this bake, so it's important to evenly distribute so that each cut square will have a few lumps. Add large lumps of the streusel topping in each corner and along all the sides and in the middle. Follow with medium sized and small lumps the same way. Finally, spread all remaining cinnamon sugar crumbs atop the cake.

Step 8

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.

Step 9

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 powdered sugar once cut, then use a spatula to remove individual pieces from the cake pan.

Step 10

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