Line an 8x4x2 or a 9x5x3 pan with foil hanging over the edge. Butter the foil and set aside.
Butter the sides of a heavy 2 quart saucepan. In pan combine sugars, milk and half and half (or light cream). Cook and stir over medium until mixture boils. Clip in a candy thermometer and reduce heat to medium low. Continue cooking at moderate rate until candy thermometer reads 236 degrees F.
Remove saucepan from heat. Add butter and vanilla, but do not stir. Cool without stirring to 110 degrees (about 50 minutes)
Remove thermometer from pan. Beat mixture vigorously with a wooden spoon until penuche just begins to thicken. Add nuts. Continue beating till penuche becomes very thick and just starts to lose its gloss (about 10 minutes).
Immediately spread penuche in the prepared pan. Score into squares while warm. When penuche is firm, use foil to lift it out of pan. Cut penuche into squares. Store tightly covered. Makes about 1 1/4 pounds.