The blowtorch called for in the last step can be either the regular type from a hardware store or a specialty chef's model from a kitchen-supply store.
Heat oven to 350 degrees. Remove short-crust dough from refrigerator. Remove parchment paper covering top, and invert onto a baking sheet. Press an 11-inch flan ring in center of short-crust dough to cut out a circle. Keeping flan ring on baking sheet, trim excess dough from outside of ring, and reserve for another use. Prick circle of dough several times with a fork. Chill until firm, about 20 minutes.
Remove dough-lined flan ring from refrigerator. Bake until light golden brown, 15 to 18 minutes. Cool completely on a wire rack before filling.
Combine creme fraiche, eggs, egg yolks, lemon zest and juice, vanilla bean, and sugar in a medium-size heatproof bowl. Place over a pot of simmering water. Cook, stirring occasionally, until mixture thickens, about 20 minutes.
Remove from heat, and stir in softened gelatin sheets. Strain mixture through a fine-mesh sieve set over a medium-size bowl. Cover surface of mixture with plastic wrap to prevent skin from forming. Set mixture in an ice bath to cool to 100 degrees.
Add butter, and using a handheld immersion blender, mix for 5 minutes. Pour into prebaked tart shell. Chill until mixture is firm, 2 to 3 hours.
Just before serving, sprinkle individual slices with a light, even layer of sugar. Caramelize sugar to a golden brown, using a blowtorch or placing under a broiler.