Line baking sheets with parchment paper. Trace 12 circles onto each sheet using the inside of the cookie cutter. Top each sheet with acetate; tape down. Chop 8 ounces semisweet chocolate; melt in a heatproof bowl set over a pan of simmering water, stirring gently, until chocolate reaches 118 degrees on a candy thermometer. Remove from heat. Chop 4 ounces semisweet chocolate; add to bowl. Stir until it cools to 84 degrees. Set bowl over simmering water. Warm, stirring, until chocolate reaches 88 degrees to 90 degrees. Transfer to bag; snip off end. Following the circles, gently pipe 1/8-inch-thick chocolate disks onto acetate.
Let disks cool for 5 minutes. While chocolate is still warm, sprinkle about 1 teaspoon tiny nonpareils onto each disk to coat. Let stand at room temperature. Repeat with 12 ounces chocolate and 6 tablespoons nonpareils to make the remaining disks.