Wiggle, wiggle, wiggle!
When checking if cakes are cooked, you'd typically be right in grabbing a skewer, sticking it into your cake and seeing if it comes out clean once removed. However, when checking a brownie, that's not gonna work! These brownies get their distinctive fudgyness from the fact that we use a minimal amount of flour, which prevents the mixture from forming a light crumb structure you'd typically associate with a cake. Instead, these are very dense, gooey treats and as such a skewer inserted into them is always going to come out similarly gooey and equally lickable!
But, Tom, I don't hear you cry, for this is the Internet, how do you check if the brownies are cooked then? Well it's simple really; give the tin a wiggle! After around 35 mins or so, if you open the oven and give the tin a gentle shake, you'll notice that the middle of the brownie will wiggle quite a lot, but the outsides will stay firm. This is a sign that the brownie is underbaked and will be too gooey to hold it's shape in the center, leading to it sinking. From here, you instead want to carry on baking it until the middle of the brownie is nearly as stable when shaken as the edge of the brownie is. A tiny, barely noticeable wobble in the center is fine, but no more than that! It should also bounce back pretty well when lightly prodded in the center with a finger, which proves that the mixture has firmed up in the oven.