1/2 cup sugar
1 tablespoons unsalted butter
1 1/2 cups roasted Marcona almonds (not in oil)
1 pound good-quality dark chocolate (62%–70% cacao), finely chopped
Coarse sea salt (for sprinkling)
Line a baking sheet with a silicone baking mat or foil.
Combine sugar with 2 tablespoons water in a small saucepan. Stir over medium-low heat until sugar dissolves. Bring to a boil and cook, occasionally swirling pan and brushing down sides with a wet pastry brush, until caramel is dark amber, about 5 minutes. Remove from heat. Immediately add butter; whisk until melted. Add almonds; stir until well coated. Transfer to baking sheet, spreading out to separate nuts. Let cool. Break up any large clumps of nuts. Set aside 1/4 of nuts.
Stir chocolate in a medium bowl set over a saucepan of simmering water until melted. Remove from heat, add nuts from baking sheet, and stir quickly to combine. Spread chocolate-nut mixture on same baking sheet, keeping nuts in a single layer. Top with the reserved nuts; sprinkle with salt. Chill until chocolate is set, about 3 hours.
Break bark into pieces and store between layers of parchment or waxed paper.