Preheat the oven to 180°C (160°C fan/Gas Mark 4) and line a 20cm square baking tin with baking parchment, leaving an overhang.
In a large bowl, whisk together 180g plain flour, 60g good quality cocoa powder, 250g caster sugar, 1 teaspoon baking powder and 0.5 teaspoon salt until combined and lump-free.
Pour in 180ml oat milk, 120ml vegetable oil and 1 teaspoon vanilla extract, then stir gently with a whisk or spatula until just combined (small lumps are fine).
Fold 100g vegan chocolate chips or chopped vegan chocolate bar into the batter.
Transfer the batter to the prepared tin, spread evenly and tap gently on the counter to release air bubbles.
Bake for 20–25 minutes, or until a skewer inserted into the centre comes out with moist crumbs but no wet batter.
Allow the brownies to cool completely in the tin on a wire rack before lifting out and cutting into squares.