Line a large baking sheet with parchment paper or a silicone baking mat; set aside.

Melt the 8 ounces coarsely chopped semi-sweet chocolate: either (a) set a heatproof bowl over a pot of simmering water (bowl should not touch the water) and stir until smooth, or (b) microwave in a microwave-safe bowl at 50% power in 20-second increments, stirring between each increment, until completely melted and smooth. Set aside.

Combine 1 cup peanut butter chips and 1 tablespoon creamy peanut butter in a small microwave-safe bowl. Microwave at 50% power in 20-second increments, stirring after each increment, until the mixture is completely smooth and melted. (Alternatively, melt over a double boiler the same way as the chocolate.)

Pour the melted semi-sweet chocolate onto the prepared baking sheet and use a rubber spatula to spread it into an even layer.

Drizzle the melted peanut butter mixture over the chocolate in parallel lines across the surface.

Use a toothpick or the tip of a knife to gently drag through the chocolate and peanut butter lines to create swirls or a marbled pattern.

Transfer the baking sheet to the refrigerator and chill until the bark is completely set, about 30 minutes.

Once hardened, remove the bark from the baking sheet and break it into pieces of your desired size.

Store bark in an airtight container at room temperature in a cool, dry place for up to 1 week. During warmer months, store the bark in the refrigerator for up to 2 weeks.