Preheat the oven to 190°C (gas mark 5) and line a 12-hole muffin tin with paper cases or grease lightly.
In a large bowl, whisk together the plain flour, whey protein powder (vanilla or unflavoured), baking powder, bicarbonate of soda and salt.
In another bowl, whisk together the eggs, milk, maple syrup, vegetable oil (or melted coconut oil) and vanilla extract until smooth.
Pour the wet ingredients into the dry ingredients and stir gently until just combined, leaving a few lumps.
Gently fold in the blueberries (unthawed if using frozen).
Divide the mixture evenly among the 12 holes, filling each about two-thirds full.
Bake for 18–22 minutes, or until the tops are golden and a skewer inserted into the centre comes out clean.
Leave the muffins in the tin for 5 minutes, then transfer to a wire rack to cool completely, or serve warm.