Brownies Cupcake

Oh, again the first-time attempt! Making brownies cum cupcake....whatever. I got this recipe from Wilton website. After browsing through all the cupcake recipes, I decided to try this one cause it is simpler and I have all the ingredients needed.

The problem is, after baking, the tops were cracked. Why? Any comment?

Anyway the recipe is as follows:

Ingredients:
1 3/4 cups all-purpose flour
1/2 teaspoon salt
4 squares unsweetened chocolate chopped
1 1/2 cups (3 sticks) butter cut into pieces
1 1/2 cups granulated sugar
4 eggs lightly beaten
2 teaspoons clear vanilla extract
2 cups pecans chopped (optional)
Makes: Each cupcake serves 1. Makes about 20 standard cupcakes or 4 Mini Tasty-Fill cakes.

Instructions
Preheat oven to 350°F.
Line standard muffin pan with baking cups.
In medium bowl, combine flour and salt.
In small heavy saucepan or microwave-safe bowl, melt butter and chocolate; stir until blended.
In large bowl, combine chocolate mixture with sugar; mix well.
Whisk in eggs and vanilla.
Add flour mixture to chocolate mixture; mix until blended.
If desired, stir in nuts.
Distribute brownie mixture evenly into baking cups.
Bake 18-20 minutes or until toothpick inserted comes out clean.
Cool cupcakes in pan on cooling rack for 5-8 minutes.
Remove cupcakes from pan; cool completely.
Any of our cupcake recipes may be prepared in our Mini Tasty-Fill pans. Spray pans with vegetable pan spray, then fill each with about 1/2 cup batter. Place pans on cookie sheet and bake 14-16 minutes. Cool in pan on cooling grid for about 8 minutes; remove from pan and cool completely before filling and icing.