Preheat the oven to 350 degrees and grease a 9”x13” cake pan or casserole dish with butter and flour.
MAKE THE TOPPING In a large bowl, combine the brown sugar, sugar, cinnamon and salt and stir to combine.
Add the butter and stir until fully combined.
Add the flour, and mix, using your hands, until fully combined and the mixture easily clumps when gently squeezed; set aside.
MAKE THE CAKE In a medium-sized bowl whisk the flour, baking powder, baking soda and salt until well combined; set aside.
In a 2-cup measuring cup, stir the milk and buttermilk until well combined; set aside.
In a stand mixer fitted with the paddle attachment, cream the butter and sugar until light and fluffy, about 5 minutes.
Add the eggs, one at a time, allowing each to incorporate before adding the next.
Add the vanilla extract and mix to incorporate.
With the stand mixer running on low, add half the dry ingredients to the egg mixture.
Slowly add the milk mixture and continue mixing until the batter is streaky before adding the remaining dry ingredients. Mix until just combined.
Transfer the batter to the prepared cake pan and, using an offset spatula, spread it evenly in the pan.
Tap the pan on the counter a few times to remove any air bubbles.
Using your hands, create clumps by gently squeezing the crumb topping, then break them up into varying sized crumbs, and sprinkle evenly over the cake batter, covering it completely.
Bake until a cake tester comes out clean when inserted into the center of the cake, for 40 to 45 minutes.
Allow the cake to cool completely in the pan.
Cut the cake into 12 portions and serve with a generous sprinkling of confectioners sugar.