Bring a large pot of salted water to a boil. Meanwhile, in a mini chopper or a small food processor, combine the peanut butter, ginger, soy sauce, rice vinegar, chile paste, and 1 tsp. of the oil. Process, scraping the sides as needed, until mostly smooth.
Heat the remaining 2 tsp. oil in a large skillet over medium-high heat. When the oil is very hot (it should be shimmering), add the shrimp and stir frequently until the shrimp start to look pink on both sides and opaque in the middle, 2 to 4 minutes. Transfer to a large bowl. Reduce the heat to medium, add the garlic and cook until softened and lightly golden, about 30 to 60 seconds. Pour 1/2 cup water into the pan and stir to loosen any browned bits on the bottom of the pan. Pour the water and the garlic into the peanut butter purée and process to blend until smooth.
Cook the noodles in the boiling water until just tender, 3 to 5 minutes (if using dried pasta, see the package for cooking times). Stir in the spinach and blanch until it softens, about 30 seconds. Drain the noodles and spinach and transfer to the bowl with the shrimp. Pour the sauce over the noodles and toss. Garnish with the scallion anad serve immediately.