01 - Preheat oven to 190°C (375°F). Line a 12-cup muffin tin with paper liners or grease lightly.
02 - In a large bowl, whisk together flour, baking powder, baking soda, and salt.
03 - In a separate bowl, combine melted butter, sugar, eggs, milk, and vanilla extract. Whisk until smooth.
04 - Pour the wet ingredients into the dry ingredients and gently fold together until just combined; do not overmix.
05 - Spoon 1 tablespoon of batter into the bottom of each muffin cup. Add 1 teaspoon of blueberry jam to the center of each cup, then cover with remaining batter, filling cups about ¾ full.
06 - Bake for 18–20 minutes, or until the muffins are golden and a toothpick inserted into the edge (not the center jam) comes out clean.
07 - While muffins bake, mix sugar and orange zest in a shallow bowl.
08 - Allow muffins to cool for 5 minutes, then brush tops with melted butter and roll in orange-scented sugar to coat.
09 - Serve warm or at room temperature.