Preheat the oven to 400 F, and position a rack in the center of the oven. Line a muffin pan with 12 paper liners. Lightly spray the liners and the top of the pan with non-stick baking spray.
In a large bowl, whisk together the flour, sugar, orange zest, corn starch, baking powder, salt and cardamom.
In a liquid measuring cup, use a fork to whisk together the milk, sour cream, eggs and vanilla extract. Whisk in the melted butter.
Add the wet ingredients to the dry, and whisk together with a fork, just until combined, but some floury bits remain.
Switch to a spatula, and briefly fold in the berries. The batter will be very thick.
Divide the batter between the muffin cups, filling them full.
Put the muffins in the oven, and immediately turn the temperature down from 400 F to 375 F.
Bake the muffins for about 22-25 minutes, until a toothpick or cake tester comes out clean.
Cool the muffins in the pan for 5 minutes, then carefully lift them out to cool on a cooling rack for 10-15 minutes.
Serve the muffins warm, with softened butter.