Heat the oven to 325 F or 165 C. Butter/spray an 8in/20cm square baking dish. Line with parchment paper as well.

Make the topping: Using a hand whisk - whisk sugars, cinnamon, salt, and butter in a medium bowl. Add flour and stir until mixture resembles a thick dough. Set aside to cool for 10-15minutes.

For the Cake: Mix flour, sugar, baking soda, and salt with an electric mixer on low speed. Add the butter one piece at a time, and beat on low until no visible butter chunks remain and the mixture is moist and crumbly. Add egg, the egg yolk, vanilla, and buttermilk. Beat on medium until light and fluffy. Transfer for baking pan and spread into an even layer.

Crumble the topping onto the cake - make them large pea sized pieces. Don't push these crumbs into the batter, just sprinkle on top and cover completely.

Bake until the crumbs are golden, and until the cake is fully cooked through, about 35-40minutes. I test it by dipping a wooden skewer or a knife into the middle of the cake and wait until it comes out clean. Dust with powdered sugar before serving! Enjoy!