Cook butter and vanilla seeds in a small saucepan over medium-high heat until dark nut brown in colour (4-5 minutes), then set aside until completely cool.
Preheat oven to 170C. Using an electric mixer, whisk eggwhites until soft peaks form, gradually add icing sugar, whisking continuously until thick and glossy. Fold in hazelnut meal, flour and orange rind, then cooled butter. Divide among 4 buttered and floured 12cm-diameter fluted tart pans and bake until golden (20-25 minutes). Stand for 5 minutes, then remove from moulds and cool on a wire rack.
For caramel fudge icing, combine ingredients in a saucepan and stir over medium-high heat until butter melts and sugar dissolves (2-3 minutes). Bring to the boil and cook until mixture reaches 114C on a sugar thermometer (4-5 minutes).Transfer to an electric mixer, add 80ml of hot water and whisk until glossy. Working quickly, spoon onto tarts and stand until set. Serve with crème fraîche.