Heat oven to 400 degrees. Grease a 12-cup muffin tin with butter and a pastry brush (alternatively use oil or liners).

Add dry ingredients (flour, oats, baking powder, cinnamon and salt) to a large bowl.

In a 4 cup glass measure (or similar sized bowl), whisk all the wet ingredients: oil, egg, honey, and milk.

Pour the wet ingredients into the dry ingredients and mix with a wooden spoon just until you don't see any dry flour. Gently fold in the 1 cup of blackberries.

Divide the batter evenly between the prepared muffin cups, making sure to get an even amount of berries in each (about 3-4 in each, depending on how big your berries are). The cups should be 2/3 to 3/4 cup full. Optional: add 1-2 more berries to each cup and/or dust the tops with a cinnamon-sugar mixture.

Bake for 15 to 18 minutes until browned on top and a toothpick has no batter on it (there will probably be berry juice, though). Rotate the pan at the halfway mark if you're using a regular oven for even browning.

Cool in pan 5-10 minutes before turning out onto a cooling rack.

When completely cool, store at room temperature in an airtight container for 1-2 days or move to the freezer for longer storage.