Bring a pot of water to a simmer. In a medium heatproof bowl, combine creme fraiche, eggs, egg yolks, lemon zest, lemon juice, and sugar. Place over simmering water. Cook, stirring occasionally, until mixture thickens, about 20 minutes.
Prepare an ice-water bath. Remove lemon curd from heat, and stir in softened gelatin. Strain mixture through a fine-mesh sieve set over a medium bowl. Cover surface of mixture with plastic wrap to prevent a skin from forming. Set mixture in ice bath to cool to 100 degrees.
Add butter, and using a handheld immersion blender, mix for 5 minutes.