In a small (1/2 quart) saucepan, add 1 cup of the sugar and 1 cup of water. Bring to a simmer, stirring to dissolve the sugar. Add the chiles and simmer for 5 minutes. Use a slotted spoon to remove the chiles to a parchment-lined plate. (The simple syrup can be used for adding heat to a cocktail.)
Turn the oven on to 350 degrees and position the rack in the lower third. Butter the bottom and sides of a 9-inch cake pan, then lay in a round of parchment paper cut to fit the pan and slather it generously with more butter (about a tablespoon). Sprinkle ½ cup of the pumpkin seeds in an even layer over the parchment, then sprinkle the seeds evenly with 2 tablespoons of the granulated sugar. Set aside.
Measure the remaining 1 ¼ cups of pumpkin seeds and 1 cup granulated sugar into a food processor. Pulse the machine until the seeds are pulverized and resemble damp sand. Add the eggs and butter and pulse until incorporated. Add the flour, baking powder and mezcal and continue to pulse, just until you have a smooth batter.
Add the chocolate and chiles to the batter and pulse until it's mixed in - two or three times should do it. Scrape the batter into the prepared pan and bake until a toothpick inserted into the center comes out clean, 35 to 40 minutes.
Let the cake cool for 10 minutes in the pan on a cooling rack, then upend it onto a serving platter and remove the parchment paper. The cake will have a crunchy layer of candied pepitas on top, which looks even more beautiful with a sprinkling of cocoa powder.