Preheat oven to 175 °C.
In a mixing bowl, whisk the butter, sugar and vanilla sugar until fluffy. Whisk in the eggs one at a time.
Combine the flour, baking powder and poppyseeds in a bowl, and carefully fold into the butter mixture. Stir in the crème fraîche and lemon juice.
Pour the mixture into a greased and lightly floured 2 litre baking tin. Bake in the lower part of the oven for approx. 50-60 minutes. Test with a wooden skewer.
Allow to cool. Mix the icing sugar with the lemon and drizzle over the cake. Garnish with finely-grated lemon zest and fresh blueberries.
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