Start by browning the butter. Add butter from the crumble ingredients and cake ingredients to a small saucepan over medium heat and melt. Once melted stir the butter constantly until most of the foam has subsided and the butter is browned and smells toasty like caramel. Pour into a heat safe bowl and let cool.
Preheat oven to 350 ℉ and line a 9x9 pan with parchment paper. To make the crumble stir together flour, sugar and cinnamon. Measure out 1/3 cup of the browned butter and add to the flour mixture. Stir until combined. It will look wet, that’s ok. Crumble will dry out, set it aside.
To make the cake whisk together the remaining browned butter, sugar, vanilla and egg until well combined. Add the flour, baking powder, salt, milk and buttermilk. Whisk just until combined, don't over mix! Pour half the batter into prepared pan and top with half of the crumble. Add remaining batter on top and use a spoon to gently spread over the crumble. Top with the remaining crumble.
Bake for 30-35 minutes or until a toothpick inserted in the center comes out clean. Let the cake cool in the pan for 10 minutes before removing from the pan to a cooling rack. Let the cake cool for another 5 minutes or so before topping with icing. Make the glaze by combining the sugar and espresso. Use a spoon or fork to drizzle over the cake. Cut into 9 or 12 squares. Enjoy!