Cake mousse layers help

[AuntyFungal]

I'm planning to bake a domed cake for DMs birthday.

How does hazelnut and browned butter sponge, blackberry mousse layers, covered in blackberry buttercream roses sound? Too much?

I'm veggie - can anyone help me with a reliable (stiff) agar agar type mousse recipe.
Do I need to either make the mousse and fridge it overnight before using it? Or slather it between the layers and fridge the whole cake? Buttercream the next day.

I fancied the slightly tart mousse layers to offset the sweet buttercream.

Well, it sounds like a bake-off technical challenge to me!

I don't know what burnt butter cake tastes like and I've never tried putting mousse in a cake so can't comment on that but making it vegetarian seems to be creating a lot of work! Honestly, I would get/make a decent tart blackcurrant jam to offset the buttercream and use it in the layers.