Chop the Chocolate

Finely process or chop the chocolate and divide it equally between a small bowl and either a microwavable bowl or the top of a double boiler.

Preheat the Oven

Twenty minutes or longer before baking, set an oven rack in the middle of the oven and preheat the oven to 350°F/175°C.

Toast and Grind the Almonds

Spread the almonds evenly on a baking sheet and bake for about 7 minutes, or until pale gold. Stir once or twice to ensure even toasting and avoid overbrowning. Cool completely. In a food processor, process until medium-fine. Divide the ground almonds equally between two small bowls.

Make the Toffee

In a heavy medium saucepan, preferably nonstick, using a wooden spatula or spoon, stir together the brown sugar, corn syrup, butter, and water. (Avoid using a silicone spatula because the long stirring time over heat can deform the spatula’s handle.) Over medium heat, bring the mixture to a boil, stirring constantly. Continue boiling, stirring often, until an instant-read thermometer reaches 285°F/140°C. Immediately remove the saucepan from the heat because the temperature will continue to rise. The correct temperature is critical to achieving a smooth, not crumbly, texture. When the temperature reaches 290°F/143°C, stir in the vanilla and baking soda.

Coat the Top of the Toffee

Pour the toffee mixture evenly into a 10 by 7 inch oval onto the prepared half sheet pan or cookie sheet, using the wooden spatula or a silicone spatula to scrape out only the toffee that is in a liquid state. Immediately scatter half of the chopped chocolate over the hot toffee. After about 5 minutes, the chocolate will be soft enough to spread with a long metal offset spatula in an even layer over the surface of the toffee. Sprinkle half of the chopped almonds evenly over the chocolate and use a small offset spatula to press them into the chocolate. Refrigerate for about 40 minutes to 1 hour, or until cool and the chocolate is set, with the surface temperature about 65°F/19°C. The almonds should barely move when pressed gently.

Carefully lift the toffee from the Silpat and flip it over.

Melt the Remaining Chocolate

In the small microwavable bowl, stirring with a silicone spatula every 15 seconds (or in the top of a double boiler set over hot, not simmering, water, stirring often—do not let the bottom of the container touch the water), heat the remaining chocolate until almost melted.

Remove the chocolate from the heat source and stir until fully melted.

Coat the Bottom of the Toffee

Pour the chocolate onto the toffee and use the long offset spatula to spread it evenly over the surface. If the chocolate hardens, wave a hair dryer over the surface to soften it so that the almonds will stick. Immediately sprinkle the remaining chopped almonds evenly over the chocolate, using a small offset spatula to press them into the chocolate.

Refrigerate the toffee again for about 30 minutes, or until cool and the chocolate has set firmly, about 62°F/17°C. Break the toffee into irregular pieces.