Heat the oven to 150 C / 300 F. Line a 33 x 46 cm / 13 x 18 inch sheet pan with parchment paper.
Melt the butter and pour it into a large mixing bowl. Whisk in the maple syrup, cinnamon, salt, vanilla, and cayenne if using.
Add the pecan halves to the bowl and toss with a wooden spoon until every piece looks glossy and coated.
Spread the pecans in a single layer on the prepared sheet pan so they roast evenly instead of steaming in a pile.
Roast for 20 to 25 minutes, stirring every 8 to 10 minutes so the syrup doesn't scorch on one side.
Watch closely in the last 5 minutes. The syrup can go from toasted to burnt fast once it starts bubbling hard.
Pull the pan when the syrup is bubbling and the pecans smell toasted and look deep brown at the edges.
Cool the pecans completely on the pan, they're still soft and sticky straight out of the oven and crisp up as they sit.
Break apart any clusters with your hands, then transfer to an airtight jar for storing.