Preheat the oven to 375 degrees and line a baking sheet with parchment paper. Arrange the almonds on the baking sheet and bake them for 10 minutes.
While the almonds are baking, place the dark chocolate pieces into a food processor bowl and pulse a few times to break the chocolate up into small pieces.
Once the almonds have been toasted and are cool enough to handle, add them to the food processor and pulse a few times until they are broken up.
Sprinkle half of the crushed almonds in a single layer on the parchment-lined baking sheet and sprinkle half of the chopped chocolate on top.
Melt the butter over medium-high heat in a heavy saucepan Once the butter melts, add the sugar and water; cook stirring occasionally.
After several minutes of cooking the sugar will start to dissolve. Once the sugar is fully dissolved and the mixture transforms into a bubbly dark-colored syrup watch the temperature closely until it reaches 290 degrees F (soft crack stage).
Once the temperature reaches 290 degrees (this can take up to 45 minutes) carefully and quickly pour the hot syrup mixture evenly over the nuts and chocolate on the baking sheet. Use a spatula after it's been poured to smooth it out.
Sprinkle the hot toffee mixture with an even layer of the chopped chocolate and then the nuts. Place the pan in the refrigerator until the toffee hardens.
Notes
Use a high-quality saucepan with a heavy bottom to prevent the butter and sugar from burning.
The size of the saucepan may seem too large for the butter and sugar, however, as the mixture heats up it will bubble up and expand in the pot.
The last stage of the syrup cooking can seem like it takes forever, be patient and don't pour the syrup until it reaches 290 degrees.
If the toffee separates during the cooking process, carefully stir in up to ¼ cup hot water a tablespoon at a time. Stir constantly until the mixture goes back together.
You can make a big batch of this homemade toffee by doubling all of the ingredients in the recipe and you will need a large baking sheet.
Test your candy thermometer in boiling water before making the toffee. If the water boils at a temperature other than 212 degrees f, adjust the temperature.