Coarsely chop the nibs and strain through a small strainer. Removing the dustier pieces will make a clearer brittle.

Toast the pumpkin seeds in a small skillet over medium heat, shaking often, until they begin to pop, 2 to 3 minutes. Remove the seeds from the skillet, and set aside.

Line a baking sheet with a Silpat or brush it with butter.

In a medium saucepan, combine the sugar, water, and cream of tartar and bring to a simmer over medium heat, stirring until the sugar dissolves. Lower the heat to medium low, cover, and let simmer for 2 to 3 minutes. Remove the pan lid and brush any sugar crystals from the sides of the pan with a wet pastry brush. Continue simmering for 3 to 5 minutes, or until the syrup begins to color. Watch closely, because the caramel can quickly burn. Test the color of the caramel by drizzling a few drops on a white plate. When the color is medium to dark amber, remove the pan from the heat and stir in the nibs, pumpkin seeds, and cayenne, if using. Working quickly, pour the mixture onto the prepared pan and spread as thin as possible with a heatproof spatula or wooden spoon.

Allow the brittle to cool completely, then break into small pieces. Store in an airtight container for up to 4 days.