Preheat oven to 400 F and move the rack to the middle position. Line a baking sheet with parchment paper or aSilpat non-stick mat.: Heat will create an immediate toasting reaction and a deep, nutty aroma as the sugars caramelize, so you should notice warm, toasty notes soon after the tray goes in. The visual cue is a slowly deepening color on the nut edges, not a rapid darkening. If your oven runs hot, rotate the tray earlier because quick browning can mean burnt edges. Avoid skipping the liner because the glaze will stick and make cleanup and even cooking harder.
Add the butter and brown sugar to a microwave-safe bowl and melt the butter. You can do this step on the stove too if you prefer. Give it a good stir.: The bowl will smell rich and caramel like as the butter and brown sugar combine, and the mixture should look glossy and free of large sugar clumps. The melt helps the sugar begin dissolving so the glaze will be smooth, and the butter prevents the sugars from drying into a grainy mess. One mistake is overheating, which can separate the fat; if that happens, let it cool slightly and whisk until cohesive.
Add the vanilla extract, maple syrup, and salt to the bowl. Continue to stir it until the sugar has pretty much dissolved (ok if it's not perfect).: The aroma will lift instantly when you add the vanilla extract and pure maple syrup , producing a warm floral-maple scent. The mixture should thin out as the syrup blends in, creating a pourable glaze. It does not need to be perfectly smooth, because oven heat will finish dissolving the remaining granules. A common misstep is stirring too aggressively and creating froth, which can burn faster in the oven.
Add the pecans to the bowl and mix until they're coated.: When the pecan halves meet the glaze you will see each nut become slick and glossy, and the kitchen will fill with toasted maple notes. Coating them thoroughly ensures every piece gains a crunchy shell when roasted, and it prevents clumping on the baking sheet. If you pile them too closely, they may steam instead of toast, so spread them out promptly after coating.
Spread the pecans out in an even layer on the lined baking sheet and bake for 8 minutes. If you're concerned about them cooking evenly on both sides, stir them halfway. I don't think it's necessary and skip this step, however.: As they roast the sugars will bubble slightly and the pecans will shift from pale to a warm golden brown, releasing a toasty fragrance and a low sizzling sound as the glaze caramelizes. The 8 minute timing gives caramelization without burning, but ovens vary, so check for even color. Avoid leaving them unattended because sugars can go from golden to bitter quickly.
Once they're done roasting, let them cool for a few minutes, then add them to a ZipLoc or sealed container to stay fresh for a few days.: Cooling lets the glaze set into a crisp shell, and during this brief rest you will hear the snap of the coating firming. Transfer to an airtight container only after they are cool, otherwise trapped steam will soften the glaze and ruin the crunch. A frequent mistake is sealing them too soon, which produces sticky, rather than crisp, nuts.