Buttermilk Chocolate cake

I am trying to make a healthy Chocolate with minimum butter. I have tweak this recipe to

160 ml butter milk

40 g butter

2 tbl rice bran oil

3 tbl coco powder

6 eggs

125g cake flour

1tsp baking powder

vanilla

Instead of a fluffy cake, it turned out like a ant nest, i wonder if it would be possible at all to bake a healthy, skinny chocolate cake that one can eat fairy often.