Sift about one - third of flour
mixture over chocolate mixture and whisk until combined; whisk in sour cream until combined, then sift remaining flour mixture over and whisk until batter is homogenous and thick.
Pour
over chocolate mixture and smooth the top.
In a small saucepan, bring the heavy cream to a simmer, then pour the cream
over the chocolate mixture.
Pour
over the chocolate mixture and let sit for 2 minutes, then whisk until smooth.
Remove the baking tray from the freezer again, pour
over the chocolate mixture and spread evenly using the back of a spoon or a spatula.