Preheat the oven to 200°C and grease a 33 x 46cm baking tray with a little butter or vegetable oil.
In a large bowl, whisk together the plain flour, caster sugar, baking powder and salt.
In another bowl, beat the egg with the milk, melted butter (or oil) and vanilla extract until combined.
Pour the wet ingredients into the dry ingredients and stir gently until just combined, leaving a few small lumps.
Spread the batter evenly in the prepared tray.
If you’re using them, scatter the blueberries or chocolate chips over the batter.
Bake for 15–20 minutes, until the edges are golden and a skewer inserted into the centre comes out clean.
Allow to cool in the tray for a few minutes, then cut into squares or rectangles using a pizza cutter or a sharp knife.
Serve warm with maple syrup and a little butter.