Line an 8×8-inch square pan with parchment, leaving overhang for easy lifting. Lightly grease the parchment.
In a small saucepan over low heat, combine the nut butter and binder. Stir until smooth, glossy, and pourable (1–3 minutes). Remove from heat and stir in vanilla and salt if using.
Add cereal to a large bowl. Pour the warm mixture over the cereal. Use a spatula to fold until every piece is coated. If adding mix-ins like seeds or dried fruit, fold them in now. If using chocolate chips, let the mixture cool 1–2 minutes first so they don't fully melt (unless melty is the vibe).
Transfer mixture to the pan. Place a second sheet of parchment on top and press firmly and evenly. Really compress the mixture—this is the difference between sturdy bars and trail mix chaos.
Chill in the fridge for 30–60 minutes, or leave at room temp for 2–3 hours, until firm. Lift out by the parchment and slice into 8–12 bars with a sharp knife.
Warm 2 tbsp nut butter and drizzle on top. Looks fancy, costs nothing. IMO, chef's kiss.