Make the cake: Preheat the oven to 325°F. Cut a round of parchment paper to fit the inside of an 8-inch round cake pan (you don't need the parchment if using a springform pan). Grease the bottom and sides of the cake pan or an 8-inch springform with the 1 teaspoon butter, line the bottom with the parchment, if using, and grease the parchment with a bit more butter. Dust the interior of the pan with the 1 tablespoon sugar, shaking and turning the pan to distribute it all around, and tap the pan to remove the excess. If using a spring form pan, wrap the bottom and sides securely in foil, making sure to tightly seal any seams so water can't seep into the pan.

Remove and discard the stems, seeds, and veins from the chipotles. Heat a medium skillet over medium-high heat, then lay the chiles in the skillet and toast them for a couple of minutes, until fragrant, pressing them flat with a spatula and flipping them frequently so they don't burn.

Transfer the chiles to a bowl and cover with very hot water. Soak until softened, 20 to 30 minutes, or longer is the chiles still have hard spots. Drain the chiles, reserving the liquid.

Put the chiles in a blender or food processor and process until you have a smooth paste, adding a bit of the soaking water as needed. Press the paste through a sieve into a bowl to make it completely smooth. Measure out 1 ½ tablespoons of the paste; if you have more than that, you can freeze the extra for another use.

Put the chocolate in a large heatproof bowl. Combine the remaining 1 cup sugar and the orange juice in a small pot and heat over the medium heat, stirring, until the sugar is dissolved. Bring the liquid just to a simmer, then remove from the heat and immediately pour over the chocolate. Let stand for about 1 minute, then stir until all the chocolate is melted (you can use a whisk, but don't beat the mixture, simply stir, as you don't want to incorporate air).

Stir the 12 tablespoons butter little by little into the still-warm chocolate mixture until melted. Add the eggs one by one, stirring until each egg is incorporated before adding the next one. Add the chipotle paste, cornstarch, and salt and stir until the batter is smooth.

Pour the batter into the prepared cake pan. Set up a water bath: Put a large deep baking dish in the oven (large enough to hold the cake pan). Place the filled cake pan in the center of the dish and carefully pour enough hot water into the baking dish (around the cake pan) to reach about halfway up the sides of the cake pan.

Bake the cake until a thin, crusty layer forms on top and the cake feels firm when gently pressed with your finger, 55 to 75 minutes.

Make the tomatillo sauce: While the cake is baking, put the tomatillos in a medium saucepan. With the tip of a paring knife, scrape the sticky seeds from the split vanilla bean and add the seeds and bean to the tomatillos, then add the piloncillo, granulated sugar, water, and cinnamon stick (the piloncillo will soften and dissolve as the sauce cooks). Bring the sauce to a simmer over medium heat and cook, stirring frequently to dissolve the piloncillo, until the tomatillos are very tender, 15 to 20 minutes. Remove from the heat.

Remove and discard the vanilla bean and cinnamon stick and, with a food processor, puree the mixture into a slightly chunky but uniform sauce. Transfer to a bowl, cover, and chill the sauce completely.

When the cake is ready, remove the water bath from the oven, remove the cake pan from the water bath, and cool the cake for about 10 minutes. If using a springform pan, remove the foil after cooling. Transfer the cake to the refrigerator and chill, covered, for at least 2 hours and up to overnight.

When ready to unmold the cake from a regular cake pan, dip a small sharp knife in hot water, dry it quickly, and run it around the edges of the cake to release it from the pan. Dip the bottom of the cake pan in very hot water for about 3 minutes and then unmold onto a cake plate; peel off the parchment. Or, if you used a springform pan, unclip and remove the sides, slide a spatula under the cake to loosen it from the base of the springform, and transfer to a serving platter.

Serve the cake at room temperature. To slice the cake, use a sharp knife dipped into hot water and dried between each slice to ensure that the sides of each piece are smooth. Serve each slice with a spoonful of tomatillo sauce.