Spread powdered sugar evenly in large nonstick skillet. Cook over medium-high heat without stirring until sugar begins to melt and caramelize on bottom (sugar layer will slide when skillet is tilted), then stir until all sugar is melted and deep amber and caramel is smooth, 3 to 4 minutes. Add 1 cup cream, rum, and 1/4 teaspoon salt (some clumps will form). Stir until caramel is thick and smooth, about 1 minute. Mix in remaining 1/4 cup cream. Remove from heat and add butter; stir until sauce is thick and smooth, about 1 minute. Strain sauce into small bowl. DO AHEAD: Sauce can be made 1 week ahead. Cool, cover, and chill. Rewarm, stirring over low heat, before using.