Boil ¼ cup water, add butter, then off heat stir in chocolate: Bring ¼ cup water to a rolling boil in a small saucepan over medium-high heat. Add butter, 1 piece at a time, whisking to combine after each addition. Remove from heat; stir in chocolate until completely melted. Set aside.

Whisk dry ingredients and combine milk, eggs, and vanilla in another bowl: Whisk together flour, sugar, and salt in a medium bowl. Whisk together milk, eggs, and vanilla in another medium bowl.

Make chocolate crepe batter and refrigerate: Gradually add milk mixture to flour mixture, whisking until smooth. Add chocolate-butter mixture, whisking until smooth. Pour through a fine sieve into an airtight container; discard lumps. Refrigerate at least 2 hours or up to overnight.

Cook one crepe: Lightly coat an 8-inch crepe pan or nonstick skillet with melted butter. Heat over medium heat until just starting to smoke. Remove pan from heat; pour about 2 tablespoons batter into pan, swirling to cover bottom. Reduce heat to medium-low; return pan to heat. Cook, flipping once, until edges are golden and center is dry, about 30 seconds per side.

Use remaining batter to cook more crepes: Slide crepe onto a plate. Repeat process with remaining batter, coating pan with butter as needed.

Make chocolate glaze: Bring cream, corn syrup, and salt to a boil in a medium-heavy saucepan over medium-high heat. Remove from heat. Add chocolate; swirl pan to cover completely with cream. Let stand about 5 minutes. Stir until smooth. Let cool completely.

Assemble cake: Place a crepe on a wire rack set over a rimmed baking sheet. Spread with about 3 tablespoons hazelnut filling. Top with another crepe. Continue layering with hazelnut filling and crepes, using about 32 crepes and ending with a crepe on top. Refrigerate until firm, about 15 minutes.

Spoon glaze and spread to coat; refrigerate: Spoon ½ cup glaze on top of the cake, spreading to edges. Spread remaining glaze around sides of cake, coating completely. Refrigerate until glaze is firm and set, about 20 minutes.

Garnish and serve: Garnish with toasted and candied hazelnuts before serving.