Line a 9x13-inch sheet pan (or similar sized baking sheet) with parchment paper and set aside.
Melt milk chocolate chips in 30-second intervals, stirring between each burst for 15–20 seconds, until the chocolate is smooth and melted. Melt in small bursts and stir very frequently to avoid burned chocolate—it’s still melting even outside of the microwave. Use a spatula to transfer chocolate from the bowl to the prepared sheet pan.
With a spatula, spread the chocolate into an even and smooth layer. Set aside at room temperature and let it set. Do not put it in the fridge because a cold chocolate layer will make the white chocolate harden too quickly.
Melt the white chocolate chips in 30-second intervals, stirring between each microwave burst for 15–20 seconds, until the chocolate is smooth. Pour the white chocolate as evenly as possible on top of the set chocolate layer and, working quickly, gently spread the melted white chocolate on top of the chocolate layer. I like to do this with a spatula or by using the back of a spoon. If the two chocolates begin swirling together, swap out for a new spoon to keep the colors separate. Immediately top the smoothed white chocolate with the M&M’s and sprinkles. Gently press these additions onto the chocolate so they stick.
Chill for 30 minutes or until firmly set. Use the parchment paper to lift the bark off the tray or out of the pan and onto a cutting board. Coarsely chop with a sharp knife using quick, decisive cuts (place a kitchen towel on top of the knife to get more leverage without hurting your palm). If the bark is getting melty or soft, return to the fridge to firm up a bit. (I love it best straight from the fridge!)
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Notes
Note 1: Using two kinds of chocolate adds depth, and any chocolate works! For less sweetness, go with semi-sweet or dark. You can also use just one chocolate. I love Ghirardelli® and Guittard® chocolate chips.Storage: Store in the fridge, especially on a hot summer day. I like keeping it in the fridge until the last minute before serving.