Pre-heat oven to 180C / 370F.
Cream together butter and sugar until fluffy.
Add eggs, one at a time, ensure each one is fully incorporated before continuing.
Stir in vanilla extract.
Stir in grated apple.
In a separate bowl, sift together the flour, baking powder and salt.
Fold in the flour mixture and milk, alternating ⅓ flour, ½ milk, ⅓ flour, ½ milk, ⅓ flour.
Fold in blueberries.
Use an ice cream scoop to divide mixture into muffin tray lined with cases.
Bake in oven for around 20 minutes, or until an inserted knife comes away clean.