In a 3 quart saucepan, gently melt the butter. Remove from heat. Add both sugars, salt and cinnamon and stir until blended. Add the flour and stir to make a soft, thick, brown dough. Set it aside to cool to room temperature while you make the cake batter.
Preheat oven to 325 degrees. Grease an 8 inch square pan and line with parchment paper if desired.
In the bowl of a stand mixer fitted with the paddle, mix flour, sugar, baking soda, and salt.
With mixer running at low speed, add softened butter one piece at a time; beating on low until mixture resembles moist crumbs, with no visible butter chunks remaining, 1 to 2 minutes.
Add egg, yolk, vanilla, and buttermilk and stir to mix. Increase mixer speed to medium high and beat until light and fluffy.
Scrape batter into prepared pan.
To assemble topping, break apart pieces of the dough in the saucepan so that you have pea sized chunks. Spread in even layer over batter, beginning with edges and then working toward center. Bake until crumbs are golden and wooden skewer inserted into center of cake comes out clean, 35 to 40 minutes.
Cool on wire rack at least 30 minutes. Before serving, dust with confectioners sugar. You can serve from the pan or if you used parchment, you can remove from the pan.