Preheat the oven to 170°C/ 338°F fan oven, 180°C/ 350°F, or gas mark 4.

Prepare and line the muffin tins with muffin cases.

Grate the zest of 2 oranges and set it aside.

Squeeze the oranges to make up to 1 cup/ 250 milliliters of orange juice and set it aside.

Melt the butter in a microwave or in a double boiler. Leave it to cool.

In a mixing bowl, place the whole wheat flour, all-purpose flour, caster sugar, baking powder, orange zest, eggs, and cooled melted butter.

Using a handheld or stand mixer, beat the butter until you get a nice smooth and thick batter.

Stir in the blueberries to the batter, leaving some berries for the topping.

Scoop the batter into each muffin case and bake for about 20 minutes or until golden brown and firm to the touch. If needed, you can test by piercing a skewer into a muffin. If it comes out clean, your muffins are ready.

Leave the muffins in their cases for about 2-3 minutes before transferring them onto a cooling rack. Or, you can serve them fresh and warm.