Make hazelnut praline. Line a baking sheet with parchment paper and spread hazelnuts on baking sheet. Add sugar to a heavy bottom sauce pot and heat over medium heat until it starts to turn amber brown. Reduce heat to low and continue to heat until all of the sugar has melted and caramelized. Immediately pour sugar over nuts. Let cool completely.

Break hazelnuts into smaller chunks and place in a food processor or high speed blender. Puree until the praline becomes a smooth paste similar to peanut butter. Set aside ¾ praline, reserving the remaining for another time.

Prepare hazelnut custard. Combine ½ cup milk with sugar and salt in a large saucepot. Bring to a simmer over low heat. Meanwhile, whisk together egg yolks, cornstarch, hazelnut praline, and remaining ½ cup milk in a heat safe bowl.

Slowly pour warm milk into the egg yolk mixture, whisking constantly. Pour the mixture back into the pot and bring to a simmer over low heat, whisking frequently. Cook for 4-5 minutes or until custard thickens. Transfer to a bowl and cover with plastic wrap directly touching the surface to avoid having a skin form. Let cool completely.

Assemble icebox cake. Lay graham crackers in the bottom of an 8x8 baking dish, breaking the crackers if necessary to create one even base. Spread ⅓ of the hazelnut filling on top followed by graham crackers, another ⅓ filling, graham crackers, and the remaining custard. Chill in the freezer for 30 minutes.

Make chocolate ganache. Place chocolate chips in a heat safe bowl. Heat heavy cream in a small pot over low heat until warm. Pour heavy cream over chocolate chips and let sit for 2 minutes. Then, stir until chocolate is melted and the ganache is smooth. Add light corn syrup, stirring until well combined. Let cool until chocolate ganache is at room temperature.

Pour chocolate ganache over icebox cake, spreading it an even layer. Chill for another 4 hours in the freezer or overnight.