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Preheat the oven to 180°C (160°C for fan ovens) and grease a 20cm square baking tin, then line it with baking parchment, leaving a slight overhang.
In a large bowl, whisk the melted butter and caster sugar together until smooth.
Beat in the eggs, one at a time, mixing thoroughly after each addition, then stir in the vanilla extract.
In a separate bowl, sift together the plain flour, cocoa powder, chocolate whey protein powder, baking powder and salt until there are no lumps.
Fold the dry ingredients gradually into the wet mixture until just combined (take care not to overmix).
Stir in the milk until the batter is uniform, then gently fold in the chocolate chips, if using.
Transfer the mixture into the prepared tin and level the surface with a spatula.
Bake for 20–25 minutes, or until a skewer inserted into the centre comes out with moist crumbs attached.
Leave to cool completely in the tin on a wire rack, then lift out using the parchment overhang.
Once cooled, cut into squares and serve. Store any leftovers in an airtight container at room temperature for up to 3–4 days.