For the elderflower buttermilk tart, mix the buttermilk with elderflower syrup. Whip the whipped cream until firm. Prepare the gelatin or agar agar according to package directions and stir a spoonful of it into the buttermilk mixture. Then mix in the remaining amount. As soon as the mixture becomes a little firmer, fold in the whipped cream.
Line a cake tin with baking paper, place the sponge cake base in it and pour in the cake mixture.
Chill the elderflower buttermilk tart well in the refrigerator for a few hours, preferably overnight.
In season, you can still decorate the cake with fresh elderflowers before serving.