Mmmmm, made this tonight and my daughters and I are gobbling it up with vanilla ice cream, hot out of the oven :) It's a fun science experiment.  Watching the cake going into the oven, my youngest daughter was doubtful it would turn into anything edible!
 
#1-1 Cup brown sugar, packed
1/4 Cup cocoa powder
1 Cup gluten-free flour (rice, etc.)
1/2 tsp xanthum gum
2 tsp baking powder
1/4 tsp. salt
#2-1/2 Cup milk
3 Tbsp. melted butter
1 egg
2 tsp vanilla
#3-1/2 Cup brown sugar
1/4 cocoa powder
#4-1 1/4 cups boiling water
 
Heat oven to 350 degrees.  Butter or spray a 2 quart casserole dish.  Mix #1 ingredients in large bowl, add #2 ingredients and stir until well mixed.  Spread into casserole dish.  Mix #3 in a small bowl and sprinkle over the top of the cake batter.  Pour boiling water over the top and bake for 20-25 minutes until starting to crack a little.  Serve warm.