Pulse flour, icing sugar, and butter in a food processor until fine breadcrumbs form.
Add vanilla seeds, then mix in eggs to form a paste and chill for 30 minutes.
Simmer gooseberries and water in a butter-smeared pan for 15 minutes.
Add sugar, dissolve it, then boil for 10 minutes until the jam reaches setting point.
Cool the jam slightly and stir in the elderflower cordial.
Preheat the oven to 180°C and grease two 12-hole petit four tins.
Line the tins with thinly rolled pastry and chill for 30 minutes.
Beat butter and sugar until light, then slowly incorporate eggs for the frangipane.
Stir ground almonds into the butter mixture.
Fill pastry cases with jam and frangipane, then bake for 15 minutes.
Decorate with flaked almonds and bake for another 15 minutes until golden-brown.
Transfer the tarts to a wire rack to cool.