Preheat your oven to 350°F (175°C) and line a 9x13 inch baking pan with parchment paper.

Mix 1 tablespoon of ground flax seeds with 2.5 tablespoons of water and let sit for 5-10 minutes to thicken.

Combine 1 cup all-purpose flour, 1 cup powdered sugar, 1/2 cup unsweetened cocoa powder, 1 teaspoon baking powder, and a pinch of salt in a large mixing bowl.

Whisk together 1 cup of unsweetened soy milk, the prepared flax egg, 1/3 cup vegetable oil, and 1 teaspoon vanilla extract in a separate bowl.

Pour the wet mixture into the dry ingredients and stir until combined, being careful not to overmix.

Transfer the brownie batter to the prepared pan, smoothing it evenly to fill the edges.

Sprinkle broken Oreo pieces over the brownie batter, pressing gently into the mixture.

Bake in the preheated oven for 30-35 minutes or until a toothpick comes out clean or with a few moist crumbs.

Allow to cool in the pan for 10 minutes before transferring to a wire rack. Slice into squares for serving.