Bring a large pot of lightly salted water to a boil. Cook pasta in the boiling water, stirring occasionally until tender yet firm to the bite, about 7 minutes. Drain and reserve 1/2 cup cooking liquid. Transfer pasta to a serving bowl.
Melt butter in a saucepan over medium heat; add onion and cook until translucent, about 3 minutes. Stir potato starch into onion mixture until dissolved, about 1 minute. Slowly pour evaporated milk into onion mixture and stir constantly. Bring sauce to a simmer and remove saucepan from heat.
Stir Cheddar cheese, mascarpone cheese, and hot sauce into sauce until smooth, adding the reserved pasta cooking liquid if needed to thin the sauce. The cheese sauce should be the consistency of gravy.
Mix cheese sauce into pasta and serve while warm and gooey.