Microwave again in 15-20 second bursts, mixing well after each burst. Repeat the process, until the chocolate has completely melted.
To a separate large microwave-safe bowl, repeat the process for the white chocolate bar or melting wafers and it has completely melted.
Pour the melted semi-sweet chocolate first over the parchment paper in the prepared pan and spread it evenly out to the edges of the pan.
Next carefully pour the white chocolate over the top of the semi-sweet chocolate and spread it in an even layer out to the edges of the pan.
Use a toothpick or tip of a small paring knife to create swirl patterns in the chocolate, as desired. Tip - Don't over-swirl, stop after it looks pretty and don't be tempted to keep going because it will tend to look muddied, not pretty, if over-swirled.
Evenly sprinkle the M&Ms over the chocolate.
Evenly sprinkle the sprinkles over the chocolate.
Place baking pan in the fridge until completely set, at least 1 hour, before breaking into pieces and serving. Bark will keep airtight at room temp for at least 2 weeks, and in the fridge for at least 1 month.
Notes
*For the chocolate, I like and recommend something like Trader Joe's Pound Plus Bars in either Milk or 72%. Bars of chocolate are easier to melt than chocolate chips and tend to melt more evenly and consistently. You can also look for chocolate 'melting wafers' which melt very smoothly.**For the white chocolate, if you can find white chocolate in bars or melting wafers, I much prefer those to white chocolate chips which can be quite resistant to melting and they also scorch easily.