Heat oil: Add oil to a medium to large skillet. (I use a 10" skillet and I can fry the whole block of tofu in one batch.) Use as much oil as needed to cover the entire bottom of the pan; don't skimp if you want it to be as crispy as possible! Heat over medium.
Add tofu: When the oil shimmers, carefully add one piece of tofu to the pan. The tofu should sizzle immediately — if not, the pan needs to preheat more. Add in the tofu pieces, doing your best to leave space between them so they don't stick together. (Careful of splattering oil. I highly recommend wearing an apron to protect your clothing, or using a splatter screen.)
Fry on one side: Let the tofu pieces fry on one side without touching for about 2-4 minutes, or until browned and crispy, then give them a flip. Resist the urge to flip them too early. The cornstarch needs time to form a crust, otherwise it may stick to the pan. When it's ready to flip, the tofu should release cleanly from the pan — if it won't, wait another minute or two.
A note on temperature: Different stoves and cookware materials all work a little differently. Adjust temperature as needed! Medium heat on my stove is typically perfect for pan-frying in a stainless or nonstick skillet, but yours may differ. Reduce heat slightly if at any point the oil starts smoking.
Flip and fry till crispy: Flip the tofu and fry again on the second side for a few minutes until crispy. At this point, go ahead and pull apart any pieces that are sticking together. Keep frying, tossing the tofu every so often, until it's evenly crispy and golden brown.
Remove from pan: If there's excess oil in your pan, you can transfer the crispy tofu pieces to a wire rack over a cookie sheet, or to a paper towel-lined plate to let any excess drip off. (Optional.) When it comes off the heat, I like to sprinkle with a little bit of extra salt while it's still hot.
Enjoy: Pan-fried tofu can be served with sauces for dipping, or tossed with veggies and sauce for stir fries. There are many other ways to enjoy too! See the recipe notes for some of my personal favorites.