Line a large baking sheet with parchment paper or a silicone baking mat. Set aside.
Melt the semi-sweet chocolate using a double broiler or in the microwave. If you melt the chocolate in the microwave, stir the chocolate every 20 seconds to help avoid seizing. Once melted and smooth, set aside.
Combine the peanut butter chips and peanut butter in a small heatproof bowl. Melt in the microwave at 50 percent power in 20-second increments, stirring after each increment, until completely smooth and melted.
Pour chocolate onto the prepared baking sheet, spreading with a rubber spatula into a smooth layer. Drizzle melted peanut butter mixture in parallel lines on top of chocolate. Run a toothpick or knife through the two layers to form swirls.
Allow the bark to set completely in the refrigerator, about 30 minutes. Once hardened, break into pieces as large or as small as you want.
Note-Store bark in an airtight container at room temperature in a cool, dry place for up to 1 week. During warmer months, store bark in the refrigerator for up to 2 weeks.