Step 1: In a large bowl, combine the masa harina, chicken bouillon granules (if using), and salt. Mix well to distribute evenly.
Step 2: Add the softened butter to the masa mixture. Using your fingertips, work the butter into the masa until crumbly.
Step 3: Gradually add 1.5 cups of hot water to the masa mixture, mixing with your hands until a dough forms. Continue to knead for 5-7 minutes, adding more hot water, a tablespoon at a time, if the dough feels too dry or crumbly. The dough should be soft, pliable, and not sticky, similar to play-doh. Cover the bowl with a damp cloth and let rest for 10-15 minutes.
Step 4: While the masa rests, prepare your cheese filling. If using blocks of cheese, shred them. If using a blend, ensure it's well mixed.
Step 5: Heat a large griddle or cast-iron skillet over medium heat. Lightly grease your hands with vegetable oil. Take a portion of the masa dough, about the size of a golf ball (or slightly larger), and roll it into a smooth ball.
Step 6: Flatten the masa ball into a thick disc, about 3-4 inches in diameter, creating a slight indentation in the center with your thumb.
Step 7: Place about 1-2 tablespoons of the shredded cheese into the indentation. Carefully bring the edges of the dough up and over the filling, pinching them together to seal the cheese completely inside, forming a ball again.
Step 8: Gently flatten the stuffed ball back into a disc, about 1/4 to 1/2 inch thick and 4-5 inches in diameter. Try to keep the edges smooth and even. If any cheese starts to peek through, pinch the masa to cover it.
Step 9: Carefully place the shaped pupusa onto the preheated, lightly oiled griddle. Cook for 4-6 minutes per side, or until golden brown spots appear and the pupusa is cooked through and slightly puffed. The cheese inside should be melted and gooey.
Step 10: Repeat with the remaining dough and cheese. Serve your homemade cheese pupusas immediately with curtido and salsa roja.