Butter sides of a heavy saucepan (2-quart size). Add sugars, half-and-half, and salt to saucepan. Cook over low heat, stirring constantly, until sugar is dissolved. Raise heat to medium and continue to cook, stirring constantly, until mixture boils. Reduce heat and continue cooking to soft ball stage*, about 234° on a candy thermometer. Remove from heat.
Add butter and vanilla, but do not stir. Cool for 5 minutes; stir in nuts. Beat with wooden spoon until candy is no longer glossy and is thickened, about 2 to 3 minutes. Quickly spoon candy onto buttered baking sheets or waxed paper. If mixture becomes to thick to drop from a spoon, add a little hot water, no more than half a teaspoon at a time. Makes about 24 pralines.