Preheat oven to 180 C. Line a 12 – cup muffin tray with paper liners.
Make the streusel: Combine the flour, sugar, cardamom powder with coconut oil, or melted butter with a fork in a small bowl. You should get a grainy mixture like wet sand. Set aside.
Mix 1 cup milk with 1 tbsp vinegar and let it sit for 5/6 minutes.
In a large bowl, combine all the flours, baking powder, baking soda, salt, sugar and crushed cardamom seeds.
In another bowl, whisk together the milk, yogurt, vanilla, and oil.
Pour the wet mixture onto the dry mixture and stir gently to combine. Fold in the diced plums.
Fill the batter evenly in the muffin cups using 2 spoons or an ice cream scoop. Sprinkle with the streusel. Place one or two pieces of sliced plum on top and press them down gently.
Bake for 20 -25 minutes or until a toothpick inserted in the center comes out clean. Transfer to a wire rack and let cool for about 10 minutes before removing from pan.