Is it a chocolate muffin you made? You can add up all the calories for the ingredients (mix, egg, oil, etc) then divide by the number of servings. Is it a chocolate muffin at a place like Starbucks or Panera Bread? It should be posted online.
Brace yourself though - muffins are terribly high in both fat and calories (unless prepared carefully at home). For example, a blueberry muffin from Starbucks has 500 calories (!) and 19 grams of fat. The lowfat version has 390 calories and 9 grams of fat.
I try to avoid most "moist" baked goods since it's the oil that keeps them all soft and moist and delicious. My sweet treat at a Starbucks is a 120 calorie biscotti
I just had to laugh, even "healthy" baked goods are ridiculous, "no sugar added" banana walnut coffee cake at Starbucks - 480 calories, 28 grams of fat!