Preheat your oven to 375°F (190°C). Line an 8x8-inch baking pan with parchment paper, letting it overhang on two opposite sides. This creates handles for lifting the bars out later.

In a large bowl, whisk together the rolled oats, brown sugar, baking powder, cinnamon, and salt. Make sure the brown sugar is free of lumps — press them out with your fingers if needed.

In a separate medium bowl, whisk the milk, egg, melted butter, and vanilla until smooth and fully combined.

Pour the wet ingredients into the dry ingredients. Stir with a rubber spatula just until no dry streaks remain. A few small lumps are fine — overmixing will make the bars dense.

Gently fold in the blueberries. If using frozen berries, do not thaw them; add them straight from the freezer to keep the batter from turning blue-gray.

Scrape the batter into the prepared pan and spread it evenly. The batter will be thick and somewhat bumpy — that's normal.

Bake for 28 to 32 minutes, until the edges are golden brown and a toothpick inserted in the center comes out clean or with just a few moist crumbs. The top should feel firm when lightly pressed.

Let the pan cool on a wire rack for at least 15 minutes. Then lift the bars out using the parchment overhang. Cut into 9 squares. Serve warm or at room temperature.