Preheat the oven to 160C/fan and grease and line 3 x 20cm cake tins. Sieve the flour and (baking powder if using).
Crack the eggs into a free standing mixer and add the sugar and vanilla. Beat for at least 5 minutes or until really thick and glossy. Once you have reached this stage very carefully fold in the flour until all incorporated. Divide equally into the greased tins and place in the oven. Bake for 20-25 minutes or until golden and, if you insert a cocktail stick in the centre of the cake, it should come out clean. Leave to cool.
In the mean time make the elderflower creme patisserie. Beat the egg yolks with the sugar until pale and fluffy. Mix the cornflower with the elderflower cordial. Bring the cream and milk to the boil and blend with the cordial. Mix the warm liquid with the egg mixture. Pour the custard back into the saucepan and simmer until the custard has thickened. Leave to cool covered so that a skin doesn’t form. Refrigerate for at least 2 hours before using or can be made the day before. Once cooled give the custard a good stir to loosen before assembling the cake.
Whip the cream until soft peaks form. Assembling the cake; Place one of the three layers and spread some of the cooled custard on the top, followed by sliced strawberries and the some whipped cream. Proceed with the next layer as before. Finnish with whipped cream and arrange the remaining strawberries on the top. I decorated my cake with elderflowers.