Lightly butter or oil the inside of a bowl or casserole dish large enough to hold the popcorn. Butter a large mixing spoon. Set aside.
Place maple syrup, in a heavy-bottomed, non-reactive saucepot over medium-high heat. Bring to a boil, reduce heat to medium low and continue boiling until the mixture reaches 236 degrees on a candy thermometer.
Place popcorn and nuts in the mixing bowl and drizzle the hot caramel over the popcorn. Working quickly, stir with oiled spoon until all popcorn is coated with caramel.
Cool and store in airtight containers or enjoy immediately!