Mother's Microwave Peanut Brittle

Mix peanuts, sugar, Karo and salt in 1/1-3 quart glass casserole. Microwave on high 8 minutes, stirring once at 4 minutes. Remove from microwave, stir in butter and vanilla. Put back in microwave and cook for 2 minutes more. Remove from microwave again and fold in baking soda. This will create the reaction, so expect to move quickly to incorporate baking soda. Then pour into a generously buttered baking sheet. Spread as thin as possible with a spatula and stretch using two forks. Cool to touch and then break into pieces.