Preheat oven to 180 degrees C / 355 degrees F.
Wash blueberries thoroughly, strain and pat completely dry. Set aside.
Chop chocolate into rough uneven chunks. Set aside.
Sift cake flour, baking powder and salt together into a bowl. Set aside.
Beat softened butter and sugar with electric mixer on medium speed until pale, fluffy and cloud-like — about 3 to 4 minutes. Do not rush this step.
Add egg and mix until fully incorporated. Scrape down sides of bowl.
Add dry ingredients in two batches alternating with milk. Fold gently with spatula only — no mixer from here.
Add vanilla extract. Fold until just combined and no dry flour is visible.
Fold in blueberries gently, then chocolate chunks.
Fill 6 muffin liners three quarters full. Press extra chocolate chunks and blueberries on top of each.
Bake at 180 degrees C for 30 minutes until golden on top. Test with toothpick inserted in centre — should come out clean.
Cool on wire rack for at least 15 minutes before serving.