Blueberry and walnut muffins

I've always been using butter in my muffins which give them a great flavour. However, after the initially crispy top comes out of the oven, it soon goes soggy as it gets cold. I came across this recipe which uses oil. I recall that oil has been a great friend to moistness in my chiffon cakes and also aids crispiness from my walnut cookies, so why not give it a try. This method from this recipe is the standard mix wet and dry but do not over-mix. However, what I like is it uses only a few bowls and involves some guess-timate (so less measuring!) which are a boon to working moms who have better things to do than standing there measuring and washing up! The original recipe uses 1 cup blueberries but I decided to sub out half a cup with walnuts.

Recipe source: https://www.inspiredtaste.net/18982/our-favorite-easy-blueberry-muffin-recipe/#itr-recipe-18982

Ingredients (Makes 10 regular or 8 ultra large muffins)
1½ C (195g) plain flour
¾ C (150g) sugar, with 1 tsp topping for each muffin (reduced from 1 tbsp in original recipe; golden caster sugar for topping works better if you have it)
2 tsp baking powder
¼ tsp salt
⅓ C neutral tasting veg oil
1 large egg
⅓ to ½ C milk (80 to 120ml)
1½ tsp vanilla extract
3 to 4 oz Blueberries, frozen or fresh (½ C)
3 to 4 oz walnuts, chopped (½ C)

Method
1. Dry: Whisk together flour, sugar, baking powder, and salt until combined. Set aside.
2. Wet: In a measuring jug, measure the oil and crack in the egg. Whisk until paler. Pour in the milk and top up to the 1 cup line on the jug. Add the vanilla extract and combine well.
3. Pour the wet slowly into the dry and be careful not to over-mix. Mix until the dry patches of flour just disappear.
4. Fold in the chopped walnuts and half the blueberries.
5. Prepare the muffin tins with liners if desired or grease each muffin cup with oil. Scoop in the batter (I used my trusty cookie scoop).
6. Dot on the reserved blueberries and push them into the batter.
7. Sprinkle on 1 tsp of sugar as evenly as possible.
8. Bake at 200 deg C for 15 minutes (for regular sized; add another 2 minutes for ultra large) until a toothpick inserted comes out clean.
9. Remove from tins and leave to cool on a baking rack.
8. To store, leave uncovered at room temperature if eaten up within 1 to 2 days. You can also freeze immediately by wrapping each one tightly in plastic then foil and freezing once they get cool.