Combine the ancho and guajillo chiles in a large bowl and add plenty of cold water to cover. Let them soak until soft, about 30 minutes. Place the soaked chiles in a blender along with the ginger, cumin, salt, and pepper. Blend until very smooth. Stop the motor a few times to scrape down the sides with a spatula and give things a stir, just to make sure everything’s getting puréed. Don’t be tempted to add liquid—you want the final sauce to be thick. Scrape the purée into a small bowl if you plan to use it soon, or into a jar with a tightfitting lid if you want to save it for later. It’ll keep in the fridge for up to 5 days, or in the freezer for 3 months.