1. Crepes: In a blender, puree flour, sugar, salt, milk, eggs, and butter until smooth, 30 seconds. Refrigerate at least 30 minutes and up to 1 day; stir for a few seconds before using.

2. Heat an 8-inch nonstick skillet over medium; lightly brush with butter. Add a scant 1/4 cup batter, tilting and swirling skillet until it evenly coats bottom. Cook until crepe is golden in places on bottom and edges lift from pan, 1 to 1 1/2 minutes. Flip and cook on other side until just set, about 45 seconds. Slide crepe onto a paper towel-lined plate. Repeat with remaining batter, coating pan with more butter as needed, and stacking crepes directly on top of one another (you should finish with about 30). Let cool completely before using, or cover and refrigerate up to 1 day.

3. Fillings: In a bowl, sprinkle gelatin over cold water. Let stand until softened, about 5 minutes. Meanwhile, melt jam in a small saucepan over medium heat until hot. Remove from heat; stir in gelatin mixture until dissolved (mixture should feel smooth when rubbed between your fingers). Transfer to a large bowl and whisk in white chocolate until smooth. In another bowl, stir together hazelnut spread and bittersweet chocolate until smooth. In a third bowl, whisk cream and confectioners' sugar to stiff peaks. Divide whipped cream evenly (about 3 cups each) between raspberry and hazelnut mixtures and stir until smooth. Refrigerate cream mixtures until thickened slightly but still spreadable, at least 1 hour and up to 2 hours.

4. To make cake, place 1 crepe on a cake plate. Spread 1/3 cup raspberry-cream mixture evenly over crepe, leaving a 1/4-inch border. Top with another crepe; spread with 1/3 cup hazelnut-cream mixture. Repeat layering, alternating fillings, until all crepes are used, finishing with a crepe. Loosely cover with plastic wrap and refrigerate until cold and set, at least 8 hours and up to 2 days. Just before slicing and serving, top cake with raspberries and dust generously with confectioners' sugar.