Place egg whites, sugar, and pinch of salt in a medium heatproof mixing bowl set over (not in) a pot of simmering water. Cook, whisking constantly, until sugar dissolves and mixture is warm to the touch, about 3 minutes. Remove bowl from heat and, using an electric mixer, beat mixture on high until stiff glossy peaks form, about 5 minutes. With a rubber spatula, gently fold egg-white mixture into ricotta mixture until combined. Divide mousse among six glasses or small bowls. Refrigerate until set, 3 hours (or up to 1 day). To serve, lightly dust with cocoa powder if desired.