Halva Diva Cake with Tahini Buttercream

Name: Halva Diva Cake with Tahini Buttercream
Cuisine: Australian
Ingredients: 5 egg whites ((150g), at room temperature); 1 pinch kosher salt; 1/8 tsp cream of tartar; 3 tbsp cold water; 1 1/4 cups (250g) superfine sugar; 1 tbsp plus 1 teaspoon cornstarch ((potato starch can be substituted)*); 1 tbsp vanilla extract; 1 tbsp vinegar (white wine, cider, or distilled); Confectioners’ sugar for dusting; 8 oz bittersweet chocolate (finely chopped); 1 cup heavy whipping cream ((dairy-free cream can be substituted)*); 2 cups (240g) confectioners’ sugar; 1/2 cup (113g) unsalted butter, room temperature ((dairy-free butter can be substituted)*); Pinch salt; 1 tsp vanilla; 1/4 cup tahini; 1 cup crushed halva; 1 cup crushed pistachios