Toast the rice powder. Put the uncooked rice in a dry skillet over medium heat. Stir or shake constantly until deeply golden brown and fragrant, several minutes. Transfer immediately to a plate to stop the toasting (it burns fast at the end). Cool, then grind to a coarse, sandy powder with a spice grinder or mortar and pestle.
Put the ground chicken and water in a skillet over medium heat. Cook, breaking the meat into small pieces, until fully cooked with no pink remaining — the chicken must reach 165°F internal temperature (verify with a thermometer). Do not brown it hard; larb chicken is typically cooked through but not seared.
Take the pan off the heat. Add the shallots and toss in the residual heat until just softened.
Add the fish sauce, lime juice, sugar (if using), and chili flakes. Toss and taste — adjust fish sauce and lime until the balance tastes right to you: salty, sour, and spicy, in that order of prominence.
Sprinkle over the toasted rice powder and toss.
Add the mint, cilantro, and green onions; toss gently. Serve warm or at room temperature with lettuce, cabbage, cucumber, and rice.