Prep your pan: Heat oven to 350°F (175°C). Grease a mini muffin tin or line with mini liners.
Soften the oats: In a large bowl, combine 1 cup rolled oats with 1/2 cup milk.
Let sit 5–10 minutes to hydrate.
Whisk the wet ingredients: Add 1/2 cup plain yogurt, 1/3 cup maple syrup, 1 large egg, 1 teaspoon vanilla, and 3 tablespoons melted butter or oil. Whisk until smooth.
Mix the dry ingredients: In a small bowl, stir 3/4 cup flour, 1 teaspoon baking powder, 1/4 teaspoon baking soda, 1/4 teaspoon salt, and 1/2 teaspoon cinnamon.
Combine: Add dry ingredients to the wet and fold gently until just combined. Do not overmix.
Add berries: Fold in 3/4 to 1 cup berries.
If using frozen, add straight from the freezer and toss lightly with 1 teaspoon flour first to reduce bleeding.
Optional brightness: Fold in 1 teaspoon lemon zest for a fresh lift.
Fill the tin: Spoon batter into the mini muffin cups, almost to the top. You should get about 24 bites.
Bake: Bake 12–14 minutes, until the tops are set and a toothpick comes out mostly clean (a few moist crumbs are fine).
Cool: Let rest in the pan 5 minutes, then transfer to a rack to cool completely. They firm up as they cool.