Preheat oven to 190'C / 375'F / Gas Mark 5. Line the tin a 9-inch square tin.
Whisk butter, eggs and sugar until pale in colour. Fold in flour gently.
Stir in milk and lemon zest.
Dust 175g of blueberries with a teaspoon of self-raising flour (to prevent them sinking to the bottom of the tin). Mix this into the batter then pour into the cake tin.
Bake in the oven for 25-30 minutes until golden. Remove from oven and leave cool in the tin.
Making the syrup
Scoop the pulp from the passion fruit into a sieve, then rub through it to extract the juice. Discard the pips.
Put icing sugar and passion fruit juice in a saucepan and heat gently until sugar dissolves.
Prick the warm cake with a fork all over, the spoon the syrup evenly over the surface.
Leave cake to cool completely in the tin, then cut into 9 squares.
To serve
Top the squares with the reserved blueberries and dust with icing sugar (pour over leftover syrup if you have any left).