To toast the almonds, preheat oven to 350 degrees. Spread whole almonds in a single layer on a baking sheet and roast 10-15 minutes, until fragrant and toasted, but being careful not to burn them. Let cool, then chop into small pieces and set aside.

Butter a baking sheet or silipat mat, then sprinkle half of the chopped almonds on the buttered surface and set aside.

In a heavy pan over medium-high heat, melt the butter, then add both types of sugar and the water. Stir to combine using a long-handled wooden spoon and bring to a boil over medium-high heat, stirring constantly. Continue stirring until a candy thermometer reaches 300 degrees (hard crack stage), usually between 10-15 minutes.

Remove from heat and immediately stir in the baking soda, working quickly. The toffee will bubble and foam a bit when reacting to the baking soda. Immediately pour the hot candy over the almonds in the prepared baking sheet and spread out into a thin layer with the back of your stirring spoon; let cool slightly for 2-3 minutes before sprinkling the chopped chocolate over the toffee. The heat of the candy will melt the chocolate after just a few minutes and then you can spread it out into an even layer with the back of spatula or knife.

Sprinkle the remaining chopped almonds over the melted chocolate and press down lightly with the back of a clean spoon. Let the toffee cool completely and the chocolate re-harden and set, then break into pieces.