Add milk and oats to a bowl, stirring to combine. Allow the oats to soak up the moisture for 20 minutes, until they get puffed up a bit. If the oats don't soak up any moisture in 20 minutes, stir and let sit another 10 minutes.

Preheat oven to 425˚F. Line a 12-cup muffin tin with parchment liners or spray with nonstick cooking spray.

Whisk together flour, baking powder, baking soda, cinnamon, salt, and nutmeg in a large bowl. Set aside.

Whisk together butter, honey, egg, and vanilla in a medium bowl. Pour the wet ingredients into the dry ingredients and stir until just combined.

Add soaked oats with milk in the bowl and blackberries. Stir until just combined. Do not overmix.

Scoop batter into prepared muffin cups, filling all the way to the top. Sprinkle with a few extra oats, if desired.

Bake for 5 minutes, then reduce temperature to 350˚F. Bake for another 15 to 17 minutes, until golden brown and cooked through.

Cool the muffins in the pan for about 5 to 7 minutes, before cooling completely on a wire rack.

Store at room temperature for up to 2 days, up to 5 days in the refrigerator. See notes about storing above the recipe card.