Put the syrup and the butter in a medium, high sided saucepan. Clip a reliable candy thermometer on the side of the pan, or have a digital thermometer ready.

Prepare a bowl of ice to cool down the pan later. I just put some ice in my sink with a little water.

Heat the syrup over medium heat until it reaches 235F. This will take in the range of 10 minutes. Do not stir, just let it boil. Be careful to catch the mixture just as it hits 235F. If you let it heat much higher you will wind up with maple candy.

When the syrup has reached 235F, take it off the heat and set it in the ice or ice water to cool to about 100F. This doesn't take long, so be alert.

Once the syrup has cooled, remove the pan from the ice and take a wooden spoon and start stirring.. You don't have to stir furiously, just stir briskly as if you were making cookie batter or something. Just keep steadily stirring the thick syrup and eventually it will start to lighten in color, and then it will magically thicken into a spreadable consistency, like peanut butter. This can take anywhere from 10 to 30 minutes, so don't get discouraged. Switch arms, pass it off to another stirrer, but keep at it. When the mixture has thickened, immediately pour it into a jar.