Instructions

1. Preheat the oven to 350°F. Line an 8 x 8-inch square pan with parchment paper on all sides and set aside.
2. First, make the crumb topping. In a medium bowl, combine the melted butter, brown sugar, flour, cinnamon and salt. Stir together until fully combined. Set aside to thicken.
3. Next, make the crumb cake. In a stand mixer fitted with the whisk attachment, combine the butter, oil, and granulated sugar. Beat on medium speed until the mixture turns pale and pearly white, about 1 minute.
4. Use a silicone spatula to scrape down the sides of the bowl. Add the egg, egg white, and vanilla extract and beat until fluffy, about 2 minutes.
5. In a separate bowl, whisk together the flour, baking powder, baking soda, and salt. With the mixer on low speed, add half the dry ingredients and mix slightly. Next add the sour cream and milk and mix slightly. Last, add the second half of dry ingredients and mix until just combined and no streaks of flour remain.
6. Use a silicone spatula to transfer the batter into the prepared pan and spread into an even layer. Squeeze the crumb topping between your palms, sprinkling small and big pieces all over the top of the cake.
   Bake until the cake is golden brown all over and a butter knife inserted into the center of the cake comes out clean, about 35 minutes. Let cool completely before dusting with powdered sugar, slicing and serving.