Mix the sugars, cinnamon, salt, and butter in medium bowl to combine.
Add the flour and stir with a rubber spatula or wooden spoon until the mixture resembles a thick, cohesive dough.
Set aside to cool to room temperature, 10 to 15 minutes.
For the cake:
Preheat the oven to 325°F.
Cut a 16-inch length parchment paper or aluminum foil and fold it lengthwise to a 7-inch width.
Butter an 8-inch square baking dish...
... and fit the parchment into the dish, pushing it into the corners and up the sides; allow the excess to overhang the edges of the dish.
In the bowl of a standing mixer fitted with paddle attachment, mix the flour, sugar, baking soda, and salt on low speed to combine.
With the mixer running at low speed, add the softened butter one piece at a time.
Continue beating until the mixture resembles moist crumbs, with no visible butter chunks remaining, one to two minutes.
Combine the egg, yolk, vanilla, and buttermilk.
Beat on medium-high speed until light and fluffy, about one minute, scraping once if necessary.
Transfer the batter to the baking pan; using a rubber spatula, spread the batter into an even layer.
Using both hands, break apart the crumb dough, rolling the broken dough between your thumb and forefinger to form crumbs about the size of large peas. Continue until all the dough has been broken down into crumbs.
Sprinkle the crumbs evenly over the cake batter, breaking apart any larger chunks. Spread the crumbs from the outside of the cake toward the center so as to not make the center too heavy.
Bake until the crumbs are golden and a wooden skewer inserted into the center of the cake comes out clean, 35 to 40 minutes. (Note: This recipe can be easily doubled and baked in a 13 by 9-inch baking dish. If doubling, increase the baking time to about 45 minutes.)
Cool on a wire rack at least 30 minutes.
Remove the cake from the pan by lifting the parchment overhang.
Dust with confectioners' sugar just before serving.
January 26, 2014 Great recipe! Easy, too. Made this for day class orientation and everyone loved it. Cook’s Illustrated.