Grind the nuts: Pulse the blanched hazelnuts in a food processor until finely ground. (Don't walk away; if you overprocess, you'll have hazelnut butter.) Transfer to a bowl and repeat with the almonds.
Mix the dry ingredients: Combine 3/4 cup of the ground hazelnuts and 1 cup minus 1 tablespoon of the ground almonds in a large bowl. Sift in the cocoa powder and confectioners' sugar.
Make the meringue: Whisk the egg whites and granulated sugar in a heatproof bowl set over a saucepan of simmering water. (Don't let the bowl touch the water.)
Continue whisking until the sugar is dissolved, about 2 minutes. (If you can still feel the sugar, keep whisking.) Turn off the heat but leave the water in the saucepan.
Using a rubber spatula, transfer the egg white mixture to the bowl of a stand mixer fitted with the whisk attachment.
Beat on medium-high speed until stiff, glossy peaks form, 5 to 6 minutes. (If you turn the bowl over, the whites shouldn't move.)
Finish the batter: Fold a few dollops of the meringue into the dry ingredients with a rubber spatula until incorporated. Gently fold in the rest until smooth and fluffy, scooping from the bottom of the bowl to the top.
Pipe into rounds: Line 2 baking sheets with parchment paper. Fit a pastry bag with a 1/2-inch plain tip and fill with the batter; press out the air. Pipe the batter into 1-inch rounds on the baking sheets, about 1 inch apart.
Rest and bake: Tap the baking sheets on the counter a few times to remove any air bubbles.
Let sit at room temperature until slightly dry, 15 to 30 minutes.
Position racks in the upper and lower thirds of the oven; preheat to 350 degrees F.
Bake 12 to 14 minutes, or until you can slide a spatula underneath the cookies, switching the position of the pans halfway through. Pull the parchment and cookies off the pans; cool.
Make the filling: Meanwhile, bring the water in the saucepan back to a simmer. Put the chocolate in a heatproof bowl and set it over the water.
Stir with a rubber spatula until the chocolate is almost completely melted. Remove the bowl from the pan.
Heat the heavy cream in a saucepan until simmering. Stir the warm cream and praline paste into the chocolate.
Fold in the butter, salt and chopped hazelnuts. Let the filling cool until spreadable, about 15 minutes.
Sandwich the cookies: Spread the filling on half of the cookies with the back of a spoon and sandwich with the remaining cookies. Refrigerate the macarons, wrapped tightly in plastic wrap, for up to 1 week.