Line a sheet pan with parchment and set it next to the stove.
Toast the pepitas in a dry 10-inch skillet over medium heat for 4 to 5 minutes, stirring, until they pop and smell nutty.
Add the maple syrup, butter, cinnamon and cayenne. Stir constantly as it bubbles hard, 4 to 5 minutes, until the syrup is thick, dark amber and clings to the seeds in a glossy sheet.
Turn off the heat and stir in the vanilla. Immediately scrape the mixture onto the parchment and spread it thin with a spatula.
Sprinkle with flaky salt and let it cool completely, 30 minutes, then break into shards.