If you use butter, the cookies tend to spread, as Debbie said. That's because butter has a very low melting point so it starts to melt, causing the cookie to spread out before the dough has a chance to set.
You can switch to butter flavored Crisco or, stick with the butter and take the following steps:
- Chill the batter before placing them on the cookie sheets.
- Chill the cookie sheets.
- Bake the cookies.
- Keep the dough in the fridge between batches
- Re-chill the cookie sheets between batches.
With the butter and the cookie sheets colder, the butter is slower to melt, giving the cookies a chance to set before they spread.
A silpat is a silicone impregnated liner for a cookie sheet that is very slippery and is used in place of parchment paper to keep baked goods from sticking.