Preheat oven to 375°F (190°C) and grease a standard muffin pan very well
Melt butter and chopped chocolate together until smooth, then cool slightly
Whisk granulated sugar and brown sugar into the chocolate mixture
Add eggs one at a time, whisking well, then stir in vanilla
Fold in flour, cocoa powder, and salt just until combined
Spoon batter halfway into each muffin cavity
Add 1 teaspoon lava filling to the center of each
Top with remaining batter until about ¾ full
Bake for 10–12 minutes until edges are set and centers look soft
Rest 2–3 minutes, loosen edges, and serve warm