Preheat oven to 275°F.
Add annatto seeds, whole black peppercorns and toasted cumin seeds to a spice grinder, buzzing into a fine powder.
Pour Mexican beer into the bowl of a food processor. Add chopped orange, garlic, cilantro and kosher salt, processing until the garlic is finely chopped. Add red pepper flakes, allspice, smoked Spanish paprika, ground ancho chile and the annatto-pepper-cumin mixture and process until a loose paste forms.
Lay a large sheet of aluminum foil on the countertop and place the pork shoulder near one of the ends (we will eventually be rolling the pork up, so leave some room to fold the foil). Slather the pork with the paste, coating every inch. Tightly roll the pork up inside the foil, folding up the sides along the way (similar to rolling up a burrito). Repeat again with another sheet of aluminum foil. Just be sure your pork is sealed tightly inside foil. Place the pork "burrito" in a baking dish then pour in a couple of inches of water (about halfway up the side of the foil-wrapped pork). Tightly cover the entire pan with another pieces of aluminum foil then carefully transfer to the oven.
Roast the pork until fall-apart tender (about 4 to 5 hours). You can test the doneness of the pork by sticking a skewer through the foil into the meat. If the skewer encounters no resistance, the pork is done. Set the roast aside to cool for at least thirty minutes before cutting into the foil.
Once the pork has cooled a bit, remove the outer layer of tin foil and cut open the inner layers. Using tongs, transfer the pork to a platter. Save the juices at the bottom of the foil packet (you want the dark, spice-flecked juices - not the water surrounding the packet) to add moisture to the pork if needed.