Cocoa beans are about 50% fat content. Much of that fat is removed during processing, becoming cocoa butter instead, in order to improve the texture and longevity of the cocoa powder. Monica mentions before drinking it that when she had tried chocolate before this, it was a lot thicker (in the light novels, she even goes as far as comparing the texture to mud) which is due to the fat not being removed. Reducing the fat makes the end product less thick, which makes it more pleasant to drink.