Preheat oven to 350 F
Line the bottom of an 8-inch spring-form pan with parchment paper. Spray the sides with cooking spray
Put the chopped chocolate and cubed butter in a bowl and pour the hot exrpesso over it. Let sit for 30 seconds or so, then stir until the chocolate and butter are melted and mixed with the coffee.
In another bowl, whisk together the dry ingredients.
Going back to the bowl with the chocolate/butter/coffee mixture, whisk in the egg, the egg yool, the sour cream, and the oil until smooth.
Pour the resulting mixture into the bowl with the dry ingredients and mix with a wooden spoon until no streaks of dry ingredients remain.
Pour into the prepared spring-form pan and bake for 28-30 minutes or until a toothpick inserted in the center comes out with no gooey batter still attached (a few crumbs are ok).
Let the cake cool on a wire rack for about five minutes before removing the spring-form ring. Let it continue to cool for about another 30 minutes on a wire rack before transferring to an 8-inch round.
Once fully cooled, cover and refrigerate (or freeze) untl ready to use.