Melt your milk chocolate on low heat, stirring until completely smooth. Stir in the peanuts, pretzels and raisins.
Line a 13 by 9 by 2-inch baking sheet with aluminum foil. Pour the melted chocolate over the foil and spread it out into a rectangle. Pop in the fridge for 1 hour to firm up.
Melt the bittersweet chocolate in another saucepan over low heat, stirring until completely smooth. Take the bark out of the fridge and pour the melted chocolate over the top. Use a spatula to smooth it out. Put back into the fridge for a couple more hours, you want it to get completely firm.
Cut the bark in 1/2 by 3-inch pieces.