How to Make It
In a saucepan, combine the grapeseed oil and walnuts and cook over low heat, stirring, until the oil reaches 300° on a candy thermometer and the walnuts are deep golden brown, 8 to 10 minutes. Strain the walnuts over a bowl, then spread on a paper towel-lined platter and let cool. Set aside 1/2 cup of the frying oil and reserve the rest for another use.
In a mortar, crush the smoked salt to a powder. Stir in the cocoa powder and sugar and season with pepper. In a bowl, toss the walnuts with the smoked cocoa seasoning.
Using a sharp knife, peel the grapefruit, removing all of the bitter white pith. Working over a small bowl, cut in between the membranes to release the sections. Reserve 1 tablespoon of juice.
In a food processor, pulse the egg yolk with the reserved tablespoon of grapefruit juice, the lemon juice and vanilla bean paste. With the machine on, slowly drizzle in the reserved 1/2 cup of frying oil until emulsified. Season the dressing with salt and pepper.
In a large bowl, toss the endives with three-fourths of the vinaigrette and season with salt and pepper. Arrange the endives on plates and top with the grapefruit sections, Manchego and cocoa walnuts. Garnish with tarragon and serve.