Heat oven to 190C/170C fan/gas 5. Grease and line a 20cm x 30cm baking tin - the lining paper should come up at least 2.5cm above the edge of the tin. In your biggest bowl, whisk the eggs and sugar for 8-10 mins until the mixture is pale and thickened.
Sift the flour and baking powder onto the egg mixture, then use a metal spoon to gently fold in. Pour this into the baking tin and bake for 35 mins until a skewer inserted into the centre comes out clean - the cake should be golden and spring back when touched. Remove from the oven and allow to stand for 5 mins before turning out onto a wire rack to cool.
Once cool, take a sharp knife and divide into 3. Mix the jam with 1 tbsp warm water and whip the cream with the vanilla essence and icing sugar until it holds its shape. Transfer one layer to a serving plate then drizzle with ¹/³ of the milk. Spread with a thin layer of jam, then cream, then scatter over 1 pack of blueberries. Cover with the second layer of sponge and then repeat with ¹/³ of the milk, a layer of jam, the cream and 2nd pack of blueberries. Put the third layer of sponge on top, drizzle over the remaining milk, then cover with cream and dot the remaining pack of blueberries on top.