I want to make a chocolate cake with walnut carmel filling. I want to frost the cake with vanilla buttercream but then pour dark choc ganache ove the frosted cake. Would the temp of the chocolate do something to the buttercream. I thought about freezing the cake first. Anyone done this who can give me pointers.

Stick the cake in the fridge until the buttercream is firm (chilled). You may then pour the ganache over as you normally would. Make sure the ganache is fairly thick or you will be able to see the white buttercream through the ganache.

You also want to make sure your bc is completely smooth. The ganache will show every imperfection. Make sure the finished cake is at room temp when you serve it otherwise the ganache will just peel off the cake.