Butter a 9x13-inch dish and a medium, heavy saucepan.

Combine 4 cups sugar and cream in the buttered saucepan. Set aside.

Pour remaining 2 cups sugar in a large, heavy skillet over medium heat. Cook, stirring constantly, until sugar begins to melt.

Place the saucepan with sugar and cream over low heat, stirring occasionally.

Continue to cook and stir sugar in the skillet until completely melted and light brown. Pour melted sugar very slowly, in a thin stream, into lightly simmering cream, stirring constantly. This step may take about 5 minutes and works best if someone pours in the melted sugar very slowly.

Heat, without stirring, to 242 to 248 degrees F (116 to 120 degrees C) or until a small amount of syrup dropped into cold water forms a firm but pliable ball. Remove from heat and stir in baking soda (mixture will foam). Drop in butter and let rest without stirring for 30 minutes.

Stir in vanilla with a wooden spoon and continue to stir vigorously until mixture loses its gloss, 10 to 15 minutes. Fold in pecans and quickly turn candy into the prepared dish. Let cool until just warm and cut into 1-inch pieces.