Why did my brownie batter separate? The recipe is:

1/2 cup          butter
8 oz             semi-sweet chocolate 
3/4 cup          granulated sugar  
1/4 cup          light brown sugar   
3                large eggs  
1 tsp            vanilla extract  
1/2 cup + 2 tbsp all-purpose flour    
2 tbsp           unsweetened cocoa powder
1/4              tsp salt


Melt the butter and chocolate over  low heat. Let cool slightly for about 10 minutes. Whisk in the sugars. Add the eggs, one at a time, and then whisk in the vanilla. Fold in flour, cocoa, and salt. Bake at 350 for 35 minutes.

I followed this recipe to the letter, but when I added in the flour, the butter starred separating out of the batter. I still followed through with the recipe, and the flavour turned out fine. However, the brownies were very crumbly and the edges very crunchy. I've never had this happen before with brownies. What could cause the butter to separate out?
There is not enough flour in this recipe, increase the amounts to 2 cups.  When using an equal amount of flour and butter you will form a short-crust type texture, this is why it was so crumbly.  It may seem counter intuitive but you actually need more flour to hold all the liquid.