When you test a fudgy brownie for doneness, the toothpick you insert in the center should come out with a few moist crumbs attached (if it's clean, it's overbaked).
A Cut Above
For neat squares, a couple of things really make a difference. Most important, cut your brownies on a cutting board, never in the pan. Also, I like using a serrated knife; first, I gently saw through the crust, and then I push straight down to cut all the way through. To keep edges extra neat, before each cut, I clean the blade under warm water and leave it slightly wet.
It's very easy to make brownies in advance. Their shelf life really depends on whether they've been cut. Store cut brownie squares in an airtight container at room temperature; they're best eaten within 1 to 2 days. A whole pan's worth, uncut and well wrapped in plastic, will keep at room temperature for up to 4 days, or in the freezer up to 3 months. Cut into squares before serving.