Before you get started, review the information below the recipe card for helpful toffee troubleshooting tips.

Preheat oven to 300°F (149°C) degrees. Line a large baking sheet with parchment paper or a silicone baking mat. (Silicone baking mat is preferred.) Spread the almonds onto the sheet and bake for 15 minutes, stirring them around twice during that time. Toasting the almonds brings so much flavor to the toffee. Remove from the oven, allow to cool, and set 1 cup aside. Chop up the other cup of almonds nice and fine. Or pulse in a food processor a few times to break them up. These will go on top of the dark chocolate.

Line a 12×17-inch jelly roll pan with a silicone baking mat or parchment paper. Set aside.

Melt the cubed butter over medium heat in a 3-quart heavy duty saucepan. Stir occasionally (about every 2 minutes) with a wooden spoon as it melts. Once melted, add the water, sugar, salt, and corn syrup. Stir constantly until the sugar dissolves, then brush down the sides of the pan with a water-moistened pastry brush. Attach a candy thermometer to the pan. Do not let it touch the bottom of the pan.

Once dissolved, stir occasionally as you bring the mixture to a boil. Once boiling, stop stirring. Rapid bubbles, a thicker consistency, as well as a slightly darker color forms around 235°F (113°C). At the 265°F (129°C, hard ball stage) point, stir in the 1 cup of toasted whole almonds. The mixture may separate when you add the nuts. If it does, temporarily remove the candy thermometer and stir vigorously until it all comes back together. Carefully reattach the thermometer and brush off any candy on the side of the pan with the pasty brush. Cook and stir the candy until it reaches 290°F (143°C, soft crack stage).

Turn off the stove, remove pan from the heat, and pour the toffee out onto the prepared jelly roll pan. Smooth into an even layer. The toffee should be thick and not spread all the way to the edges of the jelly roll pan. Allow the toffee to cool for 5 minutes. Sprinkle half of the chopped chocolate on top. Allow the chocolate to soften and melt from the heat of the toffee, then spread the melted chocolate into a thin, even layer. Sprinkle half of the crushed almonds onto the wet chocolate and press down gently with the back of a spatula to adhere them. Refrigerate for 20 minutes to set the chocolate.

Once set, flip the toffee over as a whole. You should be able to just peel it off the silicone baking mat. Melt the remaining chopped chocolate in the microwave, in 10 second spurts, stirring after each until completely smooth. Spread over the toffee and top with remaining chopped almonds, pressing down gently with the back of a spatula to adhere them. Sprinkle the top with sea salt. Refrigerate toffee again to set the chocolate, about 20 minutes. Once set, slice with a sharp knife or break into pieces—as large or small as you want.