Preheat the oven to 350ºF (180ºC). Line an 8" square cake pan with parchment paper, leaving a couple of inches of overhang to serve as handles.

In a medium-size bowl, whisk together the flour, both sugars, cocoa powder, and salt until combined. Work the butter into the flour mixture with your fingertips until it looks like dirt, with no large pieces of butter left. Press the mixture together to form chunks. Set aside in the fridge until ready to use.

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 and both sugars 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 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. Gradually beat in the coffee just until smooth and combined. Fold in the chocolate chips.

Transfer the batter to the cake pan. Top with the crumb topping. Bake until a toothpick inserted into the center of the cake comes out with a few moist crumbs attached (not wet batter), about 28-37 minutes. Cool the cake in the pan for 15 minutes, then transfer it to a cooling rack. Sprinkle with powdered sugar.