Although it's possible to use milk chocolate in some brownie recipes, it's not advisable for these Cadbury creme egg brownies. Milk chocolate is sweeter than dark chocolate, which affects the sugar level in the batter. This recipe has been developed using dark chocolate and may give quite different results if milk chocolate is used instead. Aside from that, the fondant in the creme eggs is very sweet, so the dark chocolate-based brownie batter is a sensible move. If you fancy milk chocolate brownies, take a look at this recipe - you could still customise with the creme egg garnish if you have a particularly sweet tooth. But you have been warned...