In a medium-size bowl, whisk together the flour, granulated sugar, brown sugar, cocoa powder and salt until well combined. Work the butter into the flour mixture with your fingertips until it looks like dirt, with no large pieces of butter remaining. Press the mixture together to form chunks. Set aside in the refrigerator until ready to use.
Cake
Preheat the oven to 350ºF (180ºC). Line an 8-inch square cake pan** with parchment paper, leaving a couple of inches of overhang to serve as handles to lift the cake out of the pan. If you want to serve the cake directly from the pan, you can skip the parchment paper and simply spray the pan with nonstick spray.
In a medium-size bowl, whisk the flour, cocoa powder, baking powder, baking soda and salt together until well combined.
In a large bowl, beat the butter, granulated sugar and brown sugar together, with an electric mixer at medium speed, until light and fluffy. Beat in the oil until combined. Beat in the egg, egg yolk, and vanilla in 3 separate additions, making sure each one is incorporated before adding the next.
On low speed, beat the flour mixture into the batter in 2 additions, alternating with half of the sour cream after each addition. Beat just until combined. Gradually beat in the hot coffee just until smooth and well combined. Fold in the mini chocolate chips.
Transfer the batter to the prepared cake pan. Top the batter evenly with the crumb topping. Bake until a toothpick inserted into the center of the cake comes with a few moist crumbs attached (not wet batter), about 28-37 minutes.
Cool the cake in the pan for 15 minutes, then remove the cake from the pan and continue cooling on a cooling rack. Sprinkle powdered sugar over the cooled cake.