Preheat the oven to 180°C/160°C fan/350°F. Grease and line 2 (or 4 if you have them) 6" round cake tins.
Using a stand mixer fitted with the paddle attachment or an electric hand mixer, cream together the butter and sugar until pale and fluffy. Make sure that your butter is at room temperature!
Beat in the eggs one by one, mixing well to combine between each one. Add a little of the flour between each egg too, a tablespoon or so will do.
Mix together the remaining flour and baking powder and add this to the butter/sugar/egg mixture. Mix in until just combined - don't over whisk or the cake will be dense.
Mix in the milk and vanilla extract. Gently fold the blueberries into the cake batter.
Divide the batter between the cake tins. If you have two tins, you'll have to do this in two batches. I find it easiest to weigh the batter and divide it that way but that's up to you. Bake the cakes for 25 minutes or until lightly golden and an inserted skewer comes out clean. Leave to cool for 5 minutes in the tins and then remove and place on a cooling rack to cool completely.