Preheat oven to 350 degrees F. Lightly grease a 9-inch springform pan and line the bottom with parchment paper.
Make the Crust: In a medium bowl, mix together the pretzel crumbs, sugar and butter with a fork until evenly moistened. Press onto bottom of the prepared pan. Bake until golden brown and set, 10 to 15 minutes. Remove from the oven and set aside.
Reduce the oven temperature to 325 degrees F.
Make the Cake: Place the chocolate and butter in a large heatproof bowl over a saucepan of simmering water; stir occasionally until completely melted and smooth. Remove from the heat and set aside.
Place the egg whites in the bowl of an electric mixer and, using a whisk attachment, whip the egg whites on medium speed until soft peaks form. Gradually add the sugar and whisk until the mixture is thick and glossy.
Add the egg yolks and salt to the chocolate mixture and stir until thoroughly combined. Working in three batches, gently fold the egg white mixture into the chocolate mixture until just combined.
Pour the batter into the prepared pan, smooth into an even layer, and arrange the pretzels on top. Bake until a toothpick inserted near the edge comes out clean and the center is just set, 25 to 35 minutes. Remove from the oven and let sit at room temperature for 10 minutes, then chill in the refrigerator for at least 3 hours before serving.