Preheat oven: Heat oven to 350°F. Line a baking sheet with parchment paper.
Toss almonds: In a medium bowl, combine the Marcona almonds with melted butter, tossing to coat evenly.
Add seasonings: Sprinkle with brown sugar, rosemary, salt, cayenne, and cinnamon (if using). Toss until everything is evenly distributed.
Spread on baking sheet: Spread the almonds in a single layer on the prepared baking sheet.
Bake: Bake for 12-15 minutes, stirring halfway through, until the almonds are golden and the sugar is caramelized. Watch carefully — they can go from perfect to burnt quickly.
Cool: Let cool on the baking sheet for 5-10 minutes. The coating will crisp up as they cool.
Serve: Transfer to a bowl and serve warm or at room temperature.