Place the semisweet chocolate and butter in a large microwave-safe bowl. Heat at half power in 30-second increments until the butter has melted and the chocolate melts when stirred. Stir until smooth. Set aside to cool.
Preheat oven to 350°F (175°C). Grease 48 mini muffin cups or line them with paper liners.
Add the eggs, sugar, and vanilla to the cooled chocolate mixture. Use a mixing spoon or an electric mixer to mix until thoroughly combined.
Stir in the flour and salt, mixing just until combined. Stir in the M&Ms.
Divide the batter among the muffin cups, filling each almost full.
Bake 12 to 15 minutes, or until a pick inserted into the center comes out with moist crumbs. Cool in the pan on a wire rack.
Notes
Store in an airtight container at room temperature up to 3 days.