Line a baking sheet with parchment paper; set aside.
In a medium heat-proof bowl set above a small saucepan of barely simmering water, melt the semisweet chocolate, stirring occasionally, until completely smooth. Pour the chocolate onto the prepared baking sheet and use an offset spatula to spread the chocolate into a 9x13-inch rectangle. Set aside.
In a small heat-proof bowl set above a small saucepan of barely simmering water, melt together the milk chocolate and peanut butter, stirring occasionally, until completely smooth. Dollop the peanut butter mixture over the surface of the semisweet chocolate, then use a toothpick to swirl it together. Refrigerate until set, about 30 minutes. Slice or break into pieces and serve. The bark can be stored in an airtight container in the refrigerator for up to 3 weeks.