Preheat oven to 350°F and line or grease muffin tray
Combine brown sugar and cinnamon in a small bowl to create crumble
Mix wet ingredients – oil, milk, eggs, vanilla – in a medium mixing bowl
Mix dry ingredients – flour, sugar, salt, baking powder – in a large mixing bowl
Pour wet ingredients into dry ingredients and whisk until just combined. The batter should be quite sticky
Fill each muffin cup halfway with batter, then press 3-5 frozen berries on top. Sprinkle with a generous layer of crumble
Fill muffin cup up to the top, then press in a small spoonful of chopped nuts on top. Sprinkle with another generous layer of crumble. Drizzle with melted margarine or butter until crumble is saturated.
Bake for 25-30 minutes until light golden and toothpick comes out clean (there may be some berry juice), then remove from oven and let cool