With mixer on medium-low, carefully and gradually stream syrup down inside of bowl into peanut butter mixture, avoiding the paddle if possible to minimize sugar splatter. As soon as all of the syrup has been added, increase speed to medium and beat just until mixture comes together in a single mass and pulls away from the sides of the bowl.
Transfer filling to a sheet of parchment paper (don’t scrape the sides of the bowl) and cover with another sheet of parchment paper. While still warm, roll out filling to create an even, ¼”-thick slab. Slide parchment onto a rimmed baking sheet and let sit at room temperature until set, about 1 hour.
Uncover filling. Using a round cutter or an upside-down shot glass with a diameter slightly smaller than the base of baking cups, punch out as many rounds of filling as you can. Save scraps for eating or discard. Cover rounds and chill until you’re ready to assemble the peanut butter cups.
Using scissors, trim baking cups to about ⅔” tall, then place in the cups of a standard 12-cup muffin pan.
Pour water into a small saucepan to come 1” up the sides and bring to a bare simmer over medium heat. Remove from heat and place next to muffin pan.
Place 5 oz. chocolate in a heatproof bowl and set over saucepan with water. Stir often until chocolate is melted and an instant-read thermometer registers 114°. Remove bowl from saucepan and add 3 oz. “seed” chocolate. Stir vigorously until most of the seed chocolate is melted and thermometer registers 82°. Place bowl back over saucepan and stir constantly until thermometer registers 90°. Chocolate should now be tempered and ready to use.
Using 2 spoons and a kitchen scale, dollop 11 g chocolate into each baking cup, then tap pan on counter to spread out chocolate. Remove filling from refrigerator and, working one at a time, gently press each round into chocolate in baking cups so they are centered and chocolate comes up the sides and is level with surface of filling. Let sit at room temperature until chocolate is set, 10–15 minutes.
Add remaining 3 oz. chocolate to tempered chocolate and set over saucepan again (return saucepan to heat if needed); stir until melted. Using a small offset spatula or spoon, spread about 1 tsp. chocolate over each peanut butter cup, making it as even as possible and anchoring to the sides of the baking cups. Tap pan on counter, working quickly so chocolate hasn’t had time to set, to smooth out surfaces. Chill until chocolate is set, at least 10 minutes. Serve at room temperature.
Do Ahead: Peanut butter cups can be made 1 week ahead. Store airtight at room temperature.