Make the toasted rice powder first. Put the raw rice in a dry pan over medium heat and shake it around for 5 to 7 minutes until it is deep golden and smells like popcorn. Grind it coarse in a spice grinder or with a mortar and pestle. It should feel like sand, not flour.
Cook the pork in the same pan over medium-high with 3 tablespoons of water, breaking it into small crumbles. No oil. The water keeps the lean pork from catching while it cooks through, about 6 minutes. Do not brown it hard; larb pork should be tender, not crusty.
Pull the pan off the heat. Stir in the fish sauce, lime juice, sugar, and chili flakes while the pork is still hot so it drinks up the dressing.
Let it cool for 2 minutes, then fold in the shallots, green onions, mint, cilantro, and most of the rice powder.
Taste. It should hit sour first, then salty, then heat. Adjust with more lime or fish sauce.
Pile into lettuce cups, dust with the remaining rice powder, and eat warm or at room temperature.