Dark Chocolate Ganache
4 ½ oz semisweet chocolate chips (2/3 cup)
4 ½ oz. 70% dark chocolate, finely chopped (1 cup)
¾ cup heavy cream
¼ cup unsweetened cocoa powder
1 tbsp sugar
¼ tsp kosher salt
1 cup heavy cream
¾ cup sugar
2 tsp unsalted butter, at room temperature
½ tsp. kosher salt
12 slices country white bread
4 tbsp. unsalted butter, melted
Make the ganache. In a heatproof medium bowl, combine both chocolates. In a medium saucepan with a candy thermometer attached, combine the cream, cocoa powder, sugar and salt. Cook over moderately low heat, whisking frequently, until mixture is smooth and whisk and registers 165 degrees, 3-5 minutes. Pour the cream mixture over the chocolate and stir until smooth. Let cool to room temperature.
Meanwhile, make the caramel sauce. In a small saucepan, warm the cream. In a medium saucepan, spread the sugar in an even layer. Cook over moderate heat, without stirring, until the sugar starts to melt around the edges, about 3 minutes. Reduce heat to low and shake pan to incorporate the dry sugar into the melted sugar. Continue cooking on low heat, swirling the pan frequently, until a light amber caramel forms, about 3 minutes longer. Slowly drizzle the warm cream, cook over moderate heat, whisking occasionally, until the caramel is smooth, about 3 minutes. Whisk the butter and salt. Transfer the caramel to a small bowl and let cool until just warm.
Make the sandwiches. Spread ¼ cup of the ganache onto 6 slices of bread. Top with remaining 6 slices of bread and butter both sides.
Heat a large nonstick skillet. Place 3 sandwiches in the skillet and cook over moderate heat until golden and crisp and the ganache is melted, about 2 minutes per side. Repeat with the remaining sandwiches. Serve the sandwiches with the caramel dipping sauce.
Do Ahead: The ganache and caramel can be refrigerated separately for 3 days. Bring to room temperature before serving.