Melt the chocolate and butter in a bowl set over a saucepan of gently simmering water, stirring until smooth. Stir in the salt, remove from the heat.

Whisk the egg and sugar in a large bowl by hand or using an electric mixer until pale yellow and foamy, 3 to 5 minutes. Whisk in the vanilla. Add the chocolate mixture and stir gently with a spatula until all the streaks are shades of brown.

Add the flour and stir gently until no traces of flour remain. Drop tablespoons of dough onto a parchment paper-lined cookie sheet in rounds, spacing 2 inches apart. If the tops are rounded, gently press them flat. Refrigerate until stiffened, at least 15 minutes and up to overnight.

Heat oven to 375 degrees.

Using a pastry bag or resealable plastic bag with a 1/2-inch hole cut in a corner, or a teaspoon measuring spoon, pipe or drop dollops of chocolate-hazelnut spread on top of the dough, leaving a border of dough uncovered. Sprinkle the tops with hazelnuts.

Bake until the cookie dough looks crackled and just dry, 8 to 10 minutes. Cool on the sheets and enjoy warm or at room temperature. These cookies taste best on the day they’re baked, but will keep for up to 2 days in an airtight container at room temperature or longer in the freezer.