In a heavy small saucepan combine 3/4 cup packed brown sugar, 1/4 cup water, and 1 teaspoon lemon juice. Bring to boiling over medium heat; reduce heat. Simmer about 10 minutes, stirring occasionally, until slightly thickened and bubbly.
Carefully stir in 1/3 cup whipping cream and 1 tablespoon butter. Cook and stir until butter is melted. Remove from heat; stir in 1/2 teaspoon vanilla. Cool sauce completely (can be served at room temperature).
Cover and refrigerate up to 3 days or freeze in moisture- and vapor-proof plastic freezer container. Seal, label, and freeze up to 3 months. Makes 1 cup (eight 2-tablespoon servings).
Thaw sauce overnight (about 12 hours) in refrigerator.