Preheat oven to 350 degrees F (175 degrees C). Grease 4 muffin tins or line with paper liners.

Combine butter and chocolate in a saucepan over medium-low heat. Cook and stir until melted, about 5 minutes; mix in coconut.

Beat eggs with an electric mixer in a bowl. Mix in sugar, flour, vanilla extract, and salt. Fold in walnuts. Fold in chocolate mixture. Spoon batter into prepared muffin tins until 2/3 full.

Bake in the preheated oven until a toothpick inserted into the center comes out clean, 18 to 20 minutes.