Note: if using unsalted butter, add 1/2 teaspoon salt in with the sugar.
1 cup butter
1 cup sugar
2 (3-oz.) packages slivered almonds
2 cups chocolate chips
1 (3-oz.) package sliced almonds
Line a large rimmed baking sheet with foil. Lightly butter the foil and set aside.
Place the slivered almonds in a single layer on a baking sheet and toast in the oven at 300 degrees for about 5-6 minutes, checking often, until the almonds are golden brown and toasted. Remove from the oven and set aside to cool.
In a medium saucepan over low heat, combine the butter and sugar and cook, stirring constantly with a heatproof spatula until the butter is completely melted and the sugar is dissolved. Don’t rush this step! It may take up to five or six minutes for the sugar to dissolve but you don't want the heat too high during this part of the process. Once the sugar is dissolved and butter melted, turn the heat up to medium and stir gently as the mixture comes to a boil. Again, this may take a few minutes. Once it comes to a boil, clip a candy thermometer to the side of the pan and continue stirring gently as it cooks. The mixture will gradually turn to darker shades of brown until it reaches the hard crack stage and should register 300 degrees on your candy thermometer. As you stir during this process, don’t scrape the sides of the pan, just gently stir in a figure eight motion.
Immediately take the roca off the stove once it hits 300 degrees and stir in the toasted, slivered almonds. Pour the candy onto the prepared baking sheet, taking care not to scrape the bottom of the pot as you pour it out. Using an offset spatula, quickly spread the toffee into an even layer on the baking sheet - it will cool quickly making it hard to spread.
Sprinkle the chocolate chips over the hot toffee and tent with foil. Let the chocolate melt for 2-3 minutes, remove the tented foil and spread the chocolate chips into an even layer. Sprinkle the sliced almonds over the top of the roca and set the candy aside to let the chocolate set and to let the roca cool completely. (You can refrigerate it to let the chocolate set up but sometimes the quick cooling from the refrigerator can cause the chocolate to bloom - which means it develops white streaks and blotches while it cools. It doesn't affect the taste but it doesn't look as pretty as smooth, shiny chocolate.)
Once cool and the chocolate is set, break into pieces.