Make the dressing: In a small bowl, combine the lime juice, red chile flakes, sugar and soy sauce and whisk until the sugar is dissolved.
Heat a large skillet over medium-high heat until hot. Add the rice and stir constantly until it’s golden with a nutty aroma, 3 to 4 minutes. Transfer to a mortar and pestle, spice grinder or small blender and grind until it is a coarse powder (you don’t want it too fine; some texture is nice). Set aside.
Wipe out the pan and return to medium-high heat. Drizzle with the oil, add the cauliflower, and season with salt. Stir until the cauliflower has softened slightly, 5 to 6 minutes. Add the lemongrass and shallot and cook until the cauliflower is tender and begins to turn golden on the edges, another 2 to 3 minutes.
Transfer the cauliflower mixture to a large bowl. Add the dressing, rice powder, makrut lime leaves, herbs and about 1 teaspoon salt. Toss to coat. Taste and add more salt if needed.
You can serve this dish in one of two ways: Place the cauliflower larb, crispy fried shallots and chile into separate bowls. Arrange the lettuce and perilla (if using) on a large platter and place the bowls around the leaves. Alternatively, layer the lettuce and perilla leaves (if using) on a serving platter and spoon the larb onto the leaves, garnishing with crispy fried shallots and chile.