Adding the hazelnut paste to chocolate neoclassic buttercream, or ganache for the frosting.
If you’re adding hazelnut paste to ganache, you can look at the peanut butter ganache in RHC (sponge cake section), substituting one butter for the other. I made the hazelnut version last year for the filling of a buche de noel, and it was great. If the hazelnut paste tastes fairly strong, you might want to scale back a little on the paste, or else use a very thin layer. When I served it to a group, some people loved the filling and thought it was perfect, others thought it was too rich and too hazelnutty. I though it was great, but felt like the roulade should have had more cake and less filling to balance it.
Can’t wait to see what you make!