In a food processor fitted with the blade attachment, combine the sugar, flour, butter, and cinnamon. Pulse until the mixture forms coarse crumbs. Alternatively, place the dry ingredients in a bowl, add the butter cubes, and rub them in by hand until crumbly. Refrigerate the crumble while you prepare the batter.

Preheat the oven to 340°F/170°C. Line two 12-cup standard muffin pans with paper liners.

In a large mixing bowl, whisk the eggs and oil until smooth. Add the sugar and buttermilk (or soured milk) and whisk again until well combined. Sift in the flour, baking powder, and salt. Stir just until the ingredients are incorporated—do not overmix.

Divide the batter between the muffin cups, filling each about halfway. Top with a few raspberries or blueberries. Spoon a little more batter over the fruit, just enough to cover it.

If using fresh blueberries, toss them with a teaspoon of flour before folding into the batter to prevent sinking. If using frozen berries, place them directly over the batter without thawing. Sprinkle the crumble generously over each muffin, pressing gently so it adheres.

Bake for 20 to 25 minutes, or until the muffins are lightly golden and a toothpick inserted in the center comes out with a few moist crumbs. Let the muffins cool in the pans for 10 minutes, then transfer them to a wire rack to cool completely.

Store in an airtight container at room temperature for up to 3 days.