Line an 8-inch square pan with parchment and lightly butter the paper.
Melt the butter in a large pot over medium-low heat.
Whisk in the cocoa until smooth and fragrant.
Add 10 oz mini marshmallows and stir until fully melted and glossy.
Remove from heat; stir in vanilla and salt.
Fold in the cereal until every piece is coated.
Gently mix in chocolate chips and the reserved cup of mini marshmallows.
Press the mixture firmly into the pan with a greased spatula.
Microwave drizzle chocolate until smooth, 20 to 30 seconds; drizzle over top and add sprinkles.
Cool until set, about 30 minutes, then cut into 16 squares.