I have a coworker who brings in a cake sometimes that sounds like what you describe. It is very tasty and is kind of almost like a cross between cake and a rice crispy treat (at least in texture). When I have asked what kind it is, all he could say is “It’s a Russian cake.”
Try a recipe for the cake part of pineapple upside-down cake. The cake is usually very coarse crumbed, and a bit on the dry side, all the better to allow it to soak up all that fruity, sugary goodness.
Substituting cornstarch for a small portion of the flour called for in a cake recipe will make it crumblier, since cornstarch adds no gluten to “toughen” the resulting cake. A low-protein pastry flour will do the same.
Now if that mango mousse was raspberry, I’d be standing in line myself.
Maybe you could ask the bakery what type of cake it is? They probably wouldn’t give you the recipe (although you could ask), but if they just told you what that type of cake is called, you would at least have a starting point for googling recipes.