Line a rimmed baking sheet with a silicone mat or parchment. Sprinkle ½ cup of the finely chopped almonds over an 9×12-inch area.

In a heavy saucepan over medium heat, melt the butter. Stir in the sugars, water, and corn syrup.

Bring to a boil, stirring gently. Clip on the thermometer and cook, stirring often, until it reaches 300°F and turns deep amber, about 12 to 15 minutes.

Remove from heat. Quickly stir in the chopped almonds, vanilla, and baking soda. It will foam.

Pour the toffee over the almonds on the sheet and spread it about ¼ inch thick.

Wait 1 minute, then scatter ¾ cup chocolate chips over the hot toffee. Let them melt for 3 minutes, then spread smooth and sprinkle with ¼ cup almonds.

Let cool until firm, about 1 hour. Flip the slab, melt the remaining chocolate, spread it on the other side, and top with the remaining almonds.

Chill 20 minutes until set, then break or cut into pieces.