My Chocolate Muffin : Cracks !!!!!

This is my first muffin attempt, It's pretty good, incredibly easy, & looks beautiful. I like the crack on top ... For the recipe I use the one from Info Kuliner Tabloid !

Recipe for Chocolate Muffin

200 gram cake flour
25 gram cocoa powder
1 table spoon baking powder (I use 1/2 table spoon double acting)
100 gram sugar
75 gram dark chocolate (compound), melted
1 egg medium size
150 cc milk
50 gram margarine/butter, melted (I use 25 gram butter & 25 gram margarine)
30 gram chocochip for the batter (I use 50 gram)
20 gram chocochip for topping

Method:

preheat oven for 180C.

(1) Mix together (dry ingridient) flour, baking powder & cocoa powder, set aside.
(2) In large bowl, put the sugar, milk, egg & melted dark chocolate,
wisk together untill sugar is dissolved.
(3) Add the dry ingredient to the sugar batter, stir evenly with spatula.
(4) Add the melted margarine/butter, stir evenlty with spatula.
(5) Add the chocolate chip, stir evenlty with spatula.
(6) Spoon batter in to muffin cups
(7) Bake for 20-25 minutes. (real recipe is 15 minutes)

Makes : 5 Big muffin or & 8 medium sized muffin

Although is a good muffin & so chocolaty, I'm still looking for other recipe ...
This recipe is too crumbly & dry for me ... Maybe becoz I baked it too long ...