I don't believe you can use regular Splenda in peanut brittle. I've seen comments in some cooking forums that doing so will produce a thick syrup, but not brittle.
There is a recipe for regular peanut brittle (not microwaved) on the Splenda website that uses Splenda Sugar Blend for Baking:
Peanut Brittle using Splenda Sugar Blend for Baking
"Peanut Brittle
Take the guess-work out of candy making. Use a candy thermometer.
Makes: 24 servings
Preparation Time: 20 minutes
Cook Time: 35 minutes
1 cup SPLENDA® Sugar Blend for Baking
1 cup light corn syrup
1/4 cup butter
1/2 cup water
2 1/2 cups unsalted peanuts
1 1/2 teaspoons baking soda, sifted
1. BUTTER a jellyroll pan or a slab of marble. Set aside.
2. COMBINE SPLENDA® Sugar Blend for Baking, corn syrup, butter, and water in a heavy 2 quart saucepan. Cook over medium-high heat, stirring constantly until SPLENDA® Sugar Blend for Baking dissolves and mixture begins to boil. Reduce temperature to medium-low; cook, stirring occasionally, until candy thermometer reaches 275° F. (about 20 minutes). Add peanuts and continue cooking until candy thermometer reaches 295° F. (about 10 minutes). Remove from heat and quickly sprinkle baking soda over mixture, stirring until blended. Pour into prepared pan or marble slab, spreading thinly. Cool and break into bite-size pieces. Store in an air-tight container."
Rusty