Preheat the oven to 350F and line an 8x8 baking pan with parchment so there is some overhang around the edges for easy removal.
Mix together the flour, Argo® Corn Starch, granulated sugar, brown sugar, cocoa powder, and salt for the crumbs together in a small bowl.
Add in the butter and rub the butter into the dry mixture until it resembles crumbs and it all sticks together when you press it. Store in the freezer until needed.
Mix together the flour, cocoa powder, Argo® Corn Starch, baking powder, salt, baking soda, and espresso powder for the cake together in a medium bowl. Set aside.
Add the butter, brown sugar, granulated sugar, and vanilla extract to a large mixing bowl and beat until light and fluffy - at least another 3 - 4 minutes.
Add the eggs and beat until well incorporated - at least another 2 - 3 minutes.
Mix the chocolate milk with the vinegar and set aside.
Mix in a third of the flour mixture into the large bowl until just combined. Add in half of the chocolate milk mixture and mix until just combined before adding half the remaining flour and then the remaining chocolate milk. Mix in the remaining flour until there are no streaks of flour.
Fold in the chocolate chips.
Pour the batter into the baking pan and take out the crumbs. Sprinkle the crumbs evenly over the batter and bake for about 45 - 55 minutes, or until a toothpick inserted in the center comes out with a few crumbs clinging to the sides.
Remove from the oven and let it cool for about 15 - 20 minutes before removing from the baking pan and leaving to cool completely on a wire rack.