1 – 1 1/2 cups fresh blueberries
2 1/4 cup (260 gr.) flour
2 tsp baking powder
1 tsp baking soda
1/4 tsp cinnamon
3/4 cup (150 gr.) light brown sugar
1/4 cup + 2 tbsp (6 tbsp) very soft butter
1/2 cup pink grapefruit juice + 2 tbsp plain yogurt (or all juice)
1/4 tsp salt
1 tsp apple cider vinegar (this isn’t necessary but makes the cake rise better)
Topping: Rub together 1/4 cup (4 tbsp) cold butter, 1/2 cup light brown sugar, 2 heaping tbsp flour and 1/3 cup chopped walnuts until a course crumb mixture is formed.
1. Beat the butter and sugar until fluffy.
2. Beat in the salt and eggs one at a time.
3. Sift or whisk the dry ingredients together and add them to the above alternately with the grapefruit juice/yogurt mixture.
4. Quickly beat in the vinegar if using (you won’t taste this secret ingredient!). Be careful not to over mix.
5. Gently fold in 1 cup of the blueberries (lightly floured).
6. Spoon the mixture into a wax paper lined cake pan with removable bottom. Smooth out the top and sprinkle over the remaining blueberries. Evenly cover the top with the crumble topping mixture.
7. Bake on the middle rack of a 180deg C (375deg F) oven for about 45-60 min. (test with a toothpick in the middle) or until done.
8. Cool for about 20 minutes and remove the outside ring of the pan and serve warm or cold.