For the caramel: Mix 2 cups sugar, 1/4 cup light corn syrup and 1/2 cup water in a small saucepan. Bring to a boil over medium-high heat, stirring just until the sugar dissolves. Cook, swirling the pan (do not stir), until the mixture is light amber and a candy thermometer registers 320 degrees, 8 to 10 minutes. Remove from the heat; slowly whisk in 1/2 cup heavy cream, then 2 tablespoons unsalted butter, 1 teaspoon vanilla extract and a pinch of salt. Return to low heat and whisk until smooth.
Stir 1 1/3 cups chopped nuts into the warm caramel; cool until the mixture is thick enough to hold its shape. Drop spoonfuls onto an oiled parchment-lined baking sheet; let set. Top with melted chocolate and sprinkle with coarse sea salt.