Mafaldine are flat noodles with ruffled edges. Pappardelle or tagliatelle can be substituted.
Slice each shrimp in half lengthwise; refrigerate.
Melt 1 tablespoon butter in a medium saute pan over medium. Add shrimp heads and shells and tomato paste and cook, stirring, until shells are pink and opaque, about 2 minutes. Pour in 1/4 cup wine and cook, stirring, until reduced, about 1 minute. Add 4 cups water, bring to a boil, and simmer 30 minutes. Strain shrimp stock through a sieve into a bowl; discard solids. (You should have about 2 1/4 cups.)
Meanwhile, bring a large pot of water to a boil; add salt. Cook pasta 2 minutes less than instructed on package, adding lemon slices during last 5 minutes. Drain.
Melt remaining 2 tablespoons butter in a large saute pan over medium. Add shallot and pepper flakes. Cook, stirring, until shallot is translucent, about 1 minute. Season shrimp with salt; add to pan in a single layer. Cook until light pink on one side, about 2 minutes. Turn; cook 1 minute more.
Pour in remaining 1/4 cup wine and cook, stirring, until reduced. Pour in shrimp stock and simmer until slightly reduced and thickened, about 2 minutes. Whisk in creme fraiche until combined.
Add pasta and lemon slices and stir to coat pasta with sauce. Remove from heat; squeeze remaining lemon half over top. Serve immediately, sprinkled with pepper flakes.