In a blender or food processor, pulse the sugar until powdery. Transfer the sugar to a medium heatproof bowl and whisk in the egg whites and salt. Set the bowl over a saucepan of simmering water and whisk gently until the sugar is completely dissolved, 5 minutes.
Transfer the warm egg-white mixture to the bowl of a standing electric mixer fitted with the whisk. Add the vanilla and beat at medium-high speed until stiff and glossy, about 8 minutes. Beat in the butter a few pieces at a time, making sure it is fully incorporated before adding more. The buttercream should be light and fluffy; if it appears runny at any time, transfer the bowl to the freezer for 5 to 10 minutes, then return it to the mixer and continue.
Beat in the hazelnut-praline paste until fully incorporated, scraping down the side and bottom of the bowl. Remove the bowl from the mixer and beat with a wooden spoon to remove any air bubbles.