Preheat oven to 325 degrees F (165 degrees C). Line 2 mini muffin tins with mini foil liners.
Place sugar, butter, and semisweet chocolate in top of a double boiler over simmering water. Stir frequently, scraping down the sides with a rubber spatula to avoid scorching, until chocolate is melted, about 5 minutes. Stir in flour and salt. Remove from heat and let cool, about 10 minutes.
Stir eggs 1 at a time into the chocolate mixture; add pecans and vanilla extract. Drop spoonfuls of batter into the prepared mini muffin tins.
Bake in the preheated oven until a toothpick inserted into the center comes out clean, about 25 minutes.