Preheat the oven to 350 F. Line a 12 cup muffin pan with paper liners. In a large bowl, whisk together flour, sugar, cocoa powder, salt, and baking soda.
Add in the eggs, melted butter, and vanilla extract, and mix until smooth. Fold in the M&Ms. Evenly divide the batter between the paper liners. Bake for 12-15 minutes.