In a large pot, bring 5 quarts of water to a boil along with 1 1/2 tablespoon kosher salt. Add the ditalini, stir briefly and note the recommended cooking time on the package. Boil until pasta is al dente, approximately 1-2 min less than stated.
Meanwhile, in a dry high- walled pan toast the Sichuan pepper over medium high heat for about a minute, moving it around constantly so it does not burn. Transfer the pepper to a mortar. Remove the pan from the heat.
Heat 3/4 c milk in the microwave on low, so it doesn't boil over, and dissolve peanut butter in it. Grind the toasted pepper coarsely and add it to the milk as well. Grate the rind of the mandarin orange, juice the citrus over the grated rind and set aside. Prepare your scallions, rinsing and slicing the green parts into 1/8 inch thin rounds, reserving the white parts for another use. Gently rub the peanuts to remove the majority of their skins and finely chop. Grate the cheddar cheese and set aside.
In the still warm pan, add the butter and when it melts add the flour. Return over medium high heat and cook, stirring constantly for a few minutes or until it barely starts to show some color. Off the heat slowly add the milk in a thin stream, stirring constantly to avoid the formation of lumps. Return the sauce back over medium high heat and stirring constantly, bring to a boil. Boil for a few minutes then take off the heat and slowly mix in half of the grated cheddar, stirring to melt and incorporate it into the sauce.
By now the pasta should be ready. Drain it and add to the sauce, along with peanut butter dissolved in milk, the remainder of the grated cheddar and the juice and zest of the mandarin orange. Stir carefully just to combine things and add the scallions.
Serve right away, garnished with the chopped peanuts.