Set a medium heatproof bowl over a saucepan with 1 inch of barely simmering water (the bowl should not touch the water). Add the semisweet chocolate chips and milk chocolate chips.
Warm, stirring gently, until about 90% melted, 5–7 minutes. Remove the bowl from heat and stir until completely smooth and glossy; the residual heat will finish melting.
Add the roasted salted peanuts and fold until every peanut is evenly coated and no dry spots remain.
Using a heaping tablespoon or small scoop, drop mounds of the mixture onto the prepared sheets, spacing about 1 inch apart. If using, sprinkle each with a pinch of flaky sea salt.
Let the clusters set at cool room temperature until firm, 45–60 minutes, or refrigerate until just set and no longer tacky, 15–25 minutes. Store airtight in a cool, dry place up to 2 weeks.