So I sat back and asked myself, "Just what do I do to get a flat sugar cookie?" and I came up with a few tips that may help you get that lovely flat decorating surface. If you're having trouble getting a good level cookie surface, or you just wanna do a little light cookie-theory reading, this one's for you!
Leaveners. You might notice that my sugar cookie recipe doesn't use a lot of baking powder (aka, a leavener). Leavener = rise, and you don't want a whole lot of rise out of your cookies. Do you have a tried and true sugar cookie recipe that you love, but the cookies come out a little too domed? Try reducing the leavener (such as baking powder) by a half to a quarter and see what happens. It should definitely help!
Dough, incorporated. Have you ever been rolling out your dough only to find little blobs of butter? They may look small, but they can mean big pains in your cookie baking future. Dough that is not thoroughly mixed will contain bits of unincorporated butter, and when baking, these bits of butter create lots of steam and seep and bubble out of your cookie, creating a misshapen mess along with it. Always mix your dough until these blobs of butter are no longer visible. But not a moment longer, unless you like tough cookies.
Sticky dough. If your dough is too sticky, you may notice that your sugar cookies aren't turning out as flat as you'd like them too. To make sure that your flour measurement is the most accurate it can be for your recipe, weigh the flour using a digital scale. Every baker should own one! As a general rule of thumb, 1 cup of all-purpose flour is around 120 grams. You'd be surprised how much of a difference it can make. Volume measurements are great for liquid, but over and under measuring dry ingredients like flour is easy to fall into. And don't be afraid to use a little flour when rolling out your dough - but not too much! Too much flour = a dry sugar cookie.
Chill out. I like to work with a chilled dough when rolling (not chilled enough that it'll break your arms or rolling pin, but chilled enough that you aren't trying to roll a gooey sticky mess) and I also chill my cookie dough in the freezer after rolling, cutting, and placing on parchment lined cookie sheets. 30 minutes usually does the trick. Some people swear by chilling, and others don't. Personally, I've tried putting my cookies in the oven straight after cutting and placing on cookie sheets, and I didn't end up with a very flat sugar cookie.
Getting rid of a little puffiness. Okay, so you followed all these tips and your cookies still came out of the oven looking a little puffy. If it's not too bad, they can be saved! Grab a plastic fondant smoother and gently press it on your sugar cookies and glide the smoother over their surface while still applying pressure. This trick only works while the cookies are still hot and fresh from the oven, but don't press down too hard - sugar cookies can be fragile things. This should get rid of any puffiness or bubbles.