Combine sugars and cream in a heavy 2-quart saucepan and bring to boil over medium heat, stirring occasionally with a wooden spoon, until mixture forms a thick syrup. Add pecans and butter and continue to cook over medium heat, stirring frequently. Remove saucepan to a heatproof surface (such as a wire rack) and let cool for 10 minutes. Use a tablespoon to drop rounded balls of the mixture onto sheet wax paper or foil, leaving about 3 inches between each ball for pralines to spread. Allow to cool.