Instructions

• Preheat the oven to 350°F (175°C). Lightly grease a 9-inch springform pan and line the bottom with parchment paper.
• Make the crust: In a medium bowl, combine the 2 cups finely crushed pretzels, 1/4 cup granulated sugar, and 2/3 cup melted unsalted butter. Stir with a fork until the crumbs are evenly moistened. Press the mixture firmly and evenly into the bottom of the prepared pan.
• Bake the crust on the middle rack until golden brown and set, 10 to 15 minutes. Remove from the oven and place the pan on a wire rack. Reduce the oven temperature to 325°F (163°C).
• Melt the filling chocolate: Place the 16 ounces chopped dark chocolate and 1/2 cup unsalted butter (cut into pieces) in a heatproof bowl set over a saucepan of simmering water (bowl should not touch the water). Stir occasionally until completely melted and smooth. Remove the bowl from the heat and let the chocolate cool slightly until warm but not hot.
• Whip the egg whites: In a clean, dry bowl (or the bowl of a stand mixer) fit with the whisk attachment, whip the 4 egg whites on medium speed until soft peaks form. Gradually add the 1/4 cup superfine sugar and continue whipping until the whites are thick and glossy.
• Combine yolks and salt with chocolate: Stir the 4 egg yolks and 2 teaspoons flaky sea salt into the warm chocolate mixture until fully incorporated.
• Fold in the egg whites: Add about one-third of the whipped egg whites to the chocolate mixture and stir to lighten it. Gently fold in the remaining egg whites in two additions, folding just until no large streaks remain (do not overmix).
• Transfer and top: Pour the batter over the baked crust in the springform pan and smooth the top with a spatula. Arrange the 1 cup mini pretzels on the surface.
• Bake the cake at 325°F (163°C) until a toothpick inserted near the edge comes out clean and the center is just set (it may still have a slight wobble), about 25 to 35 minutes.
• Cool and chill: Remove the cake from the oven and let it sit at room temperature in the pan for 10 minutes. Then refrigerate the cake (in the pan) for at least 3 hours before serving.