Preheat your oven to 350ºF and prepare an 8x8in square pan with parchment paper.

In the bowl of a stand mixer with the paddle attachment, cream together the butter and sugars until fluffy.

Add the egg and beat until combined. Add in the vanilla and oil and continue beating until fully incorporated.

In a separate bowl, mix together the dry ingredients.

Add in the dry ingredients, alternating with the milk, until the batter is just combined. Don't over-mix!

Fold in the blueberries gently until spread throughout the batter evenly.

Spread the batter into your prepared pan until it's level.

In a small bowl, mix together the streusel ingredients until the mixture resembles wet sand and crumbles in your fingers. Sprinkle the streusel on top of the batter in the pan evenly.

Bake the bars for 30-35 minutes, or until a toothpick comes out clean. For dark pans, bake at 325ºF for about 40 minutes.

Let the bars cool slightly in the pan before removing. Cut into slices and enjoy!