Combine flour, cocoa powder, baking powder, baking soda, and salt in a medium bowl and set aside. In medium saucepan over medium heat add in butter, chocolate, and espresso powder. Melt together while stirring. Remove melted chocolate from stove and add in 1 teaspoon vanilla extract, brown sugar, and sour cream. Mix until combined then add in egg yolk and vigorously whisk. Add wet ingredients to the dry and stir until fully combined.