1. Preheat oven to 350 degrees. Coat a standard 12-cup muffin pan with cooking spray; set aside. In a medium bowl, whisk together flour, baking powder, salt, and millet. Working over the bowl, toss the cranberries in a medium bowl with about 2 teaspoons of the flour mixture to lightly coat; set aside flour mixture and cranberries.

2. In the bowl of an electric mixer fitted with the paddle attachment, beat the butter and sugar on medium-high speed until light and fluffy, about 3 minutes. Add the eggs, one at a time, beating until combined. Mix in vanilla.

3. With the mixer on low speed, add the reserved flour mixture, beating until just combined. Add milk, beating, until just combined; do not overmix. Using a rubber spatula, fold in cranberries. Divide batter evenly among prepared muffin cups.

4. Bake, rotating the pan halfway through, until the muffins are golden brown and a cake tester inserted in the center of one muffin comes out clean, about 30 minutes. Transfer pan to a wire rack to cool 10 minutes. Turn muffins on their sides in their cups, and let cool. Serve warm or at room temperature.