Brownie with chocolate mousse on top
Brownies are thicker than a standard cake sponge, but either can absorb moisture, if you want to prevent that then a thin layer of chocolate frosting would work fine, or as you suggest a thin layer of chocolate. 

It is opinion based whether I think a brownie or a sponge would be better with mousse, what I will say is that the consideration is the difference in consistency between the two. A rich, soft mousse on top of soft, fluffy cake may more people than hitting a thick, harder layer of brownie, but it all depends on the effect you are looking for.