Whisk together sugar, cornstarch and salt in a medium pot. Whisk in the egg yolks and milk until fully combined. Add in the chopped chocolate.
Turn on the heat to medium and cook the mixture, whisking constantly, and occasionally taking a spatula and scraping up any thickened pudding in the edges of the pan. Cook until the pudding has completely thickened and large bubbles are forming and popping, about 8 minutes.
Remove from heat; immediately pour through a fine mesh strainer into bowl, using a spatula to push all the pudding out while leaving behind any bits of cooked egg.
Whisk the butter and the extract into the pudding until fully combined.
Pour the pudding into the serving dish or dishes. Let cool at room temperature before chilling in the refrigerator for at least 3 hours or overnight.
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