Line a large rimmed baking sheet with parchment paper.
Put chopped chocolate in a shallow, medium heatproof bowl. In a medium saucepan, bring 1/2 in. water to a bare simmer. Set bowl over pan and heat chocolate, stirring often, until melted and registers about 115° on a digital or instant-read thermometer, 3 to 7 minutes.
Remove bowl from water and dry underside. Let chocolate cool (at first, it will rise about 5°), stirring constantly, to 95°, 5 to 12 minutes. Add large chocolate pieces and stir constantly until chocolate cools to 88° to 90°, 2 to 3 minutes. Spoon out unmelted chocolate and save for other uses.
Pour melted chocolate onto center of parchment and spread with an offset spatula or flexible scraper to about 1 in. from pan edges. Sprinkle with peppermint candies and gently pat them into chocolate.
Set pan on a cooling rack and let stand at room temperature until chocolate is firm, about 1 1/2 hours. Break bark into pieces.
Make ahead: Up to 1 month, airtight at room temperature.