New Orleans Praline Pieces

Butter a 9-inch square metal baking pan. Sift granulated sugar through a sieve into a bowl to remove any lumps or large crystals, then rub brown sugar through sieve into bowl. Pour sugars into a 2 1/2- to 3-quart heavy saucepan, being careful not to get sugar on side of pan. Add cream, butter, and 1/4 teaspoon salt and cook over very low heat, stirring frequently with a wooden spoon and washing down any sugar crystals on side of pan with a pastry brush dipped in cold water, until sugar is dissolved (do not let simmer), 10 to 15 minutes. Clamp on candy thermometer, then boil syrup over moderately high heat, undisturbed, until it registers 236 °F and a teaspoon of syrup dropped into a small bowl of cold water holds a very soft ball when pressed between your fingers, 3 to 6 minutes. Remove pan from heat, leaving thermometer in place, and cool, undisturbed, until syrup registers 220 °F, 1 to 3 minutes. Stir syrup with cleaned and dried wooden spoon until thickened and creamy, 1 to 2 minutes, then immediately stir in pecans. Working very fast (syrup hardens quickly), pour into baking pan, scraping sides of saucepan with wooden spoon. Let mixture harden at room temperature, about 45 minutes. Cut and break into pieces.