Clip a candy thermometer to the side of the pot. Continue boiling, without stirring, until temperature reaches 295 degrees, about 6 Minutes. Using a candy thermometer is essential because even a few degrees can make the difference between brittle that hardens properly and overcooked burnt caramel. When the sugar begins to brown, add the nuts and stir gently to ensure even cooking. Once the nuts are coated with the sugar mixture remove saucepan from heat, and stir in the butter or margarine and baking soda; the mixture will begin to foam up, so mix quickly and carefully. Once it is fully combined, pour the mixture onto the prepared baking pan. Using a greased spatula spread the mixture as thinly as possible on the pan.