Preheat the oven to 350ºF (180ºC). Line a 9-inch square baking pan with parchment paper, leaving a couple of inches of overhang. (If you would like to serve the cake directly out of the pan, skip the parchment paper and grease the inside of the pan, then sprinkle with a mixture of equal parts flour and cocoa powder. Tap out the excess.)
In large bowl, whisk the flour, cocoa powder, cornstarch, baking powder, baking soda and salt together until well blended. Stir in the granulated sugar and brown sugar.
In a medium-sized bowl, whisk together the oil, egg, egg yolk, vanilla extract and sour cream until smooth and well blended. Add this mixture to the dry ingredients and beat, with an electric mixer at medium speed, until combined. Gradually mix in the hot coffee and and beat until the batter is smooth.
Transfer the batter to the baking pan. Bake until a toothpick inserted into the center of the cake comes out mostly clean or with a few moist crumbs attached, about 18-25 minutes. Cool the cake in the pan for 15 minutes, then remove the cake and transfer it to a cooling rack.