A rich cream cheese filling adds a twist to ordinary coffee cake—unless it breaks away and sinks to the bottom of the pan. This brunch staple is fraught with pitfalls, from dry, bland cake to lackluster fillings that sink to the bottom. Rich sour cream contributed moisture to our cake as well as a subtle tang—a perfect backdrop for the cream cheese swirl. For the filling, we settled on a base mixture of softened cream cheese and sugar and then added lemon juice to cut the richness and a hint…