Pre-heat your oven to 180 degrees
In a large bowl, cream the butter and sugar together until fluffy
Add the eggs, flour, baking powder, vanilla essence and lemon zest. Mix until smooth
Coat your blueberries in around 2 tbsp of flour (this will ensure they don't all sink to the bottom of your cake whilst cooking). Then, fold them into your batter
Pour your mixture into a loaf tin and bake for 35-45 minutes. Insert a skewer into your cake - if it comes out clean, you're done!