I make these successfully all the time, using frozen berries, also using the tulip paper cases. I have never placed them in the fridge before baking as the frozen fruit needs to stay frozen to stop bleeding of juice.
Mixed Berry Buttermilk Muffins:
375 gm SR flour
165 gm caster sugar
1 egg, lightly beaten
1 tsp vanilla extract
160 ml vegetable oil
180 ml buttermilk
200 gm frozen mixed berries (or a single variety, I like raspberries best)
Preheat oven to 190C.
Line a 12 hole muffin tin with paper cases.
Sift flour and sugar, mix in remaining ingredients.
Divide mixture between cases (I use an ice cream scoop).
Bake for about 20 minutes.
Cool for 5 minutes then then place on wire rack and dust with icing sugar.
Do not over-mix, it should be slightly lumpy.
The muffins freeze well.