Butter a 7x11-inch pan (or 9x12 for slightly thinner roca). Sprinkle 1 cup of toasted almonds on bottom of the pan.
In a heavy saucepan at medium-high heat melt butter and add brown sugar. Stir until gently boiling. Reduce heat to medium or medium-low and boil 12 minutes exactly, stirring constantly. *Edited to add* I make mine without a candy thermometer with success, but since some readers have noted having trouble with this recipe- you might wish to stick an instant read thermometer in there just to make sure it's about the right temperature. Pull it off the heat when it has reached the hard-crack stage (300 degrees F)- or when the mixture starts to separate (the oil starts to separate from the sugar). If it gets higher than 320, it will not remain solid.
Remove the mixture from heat, give it a good stir to mix up that butter and sugar one more time, and immediately pour the hot mixture into the pan over the almonds. (see *Recipe Notes below) Place the chocolate bars on top. Let them sit a minute or so until melted, then spread chocolate around carefully. Sprinkle with the remaining toasted almonds, and gently press them into the chocolate.
Cool completely, and then break apart into chunks with a sharp knife. Store in a covered container.
TIPS: This recipe is gluten-free adaptable- just be sure to use a brand of chocolate that is known to be GF.
If you like the sweet-salty combination in desserts, use roasted almonds that are salted (then you can chop them and toast them lightly in a dry frying pan). If you don't care to have much salt, use unsalted natural almonds. I went for the salty kind and LOVED it.
If you don't have a 7x11-inch pan, it's okay to use a 9x12-inch pan. You probably won't want to go all the way to the edges with the almonds, toffee and chocolate or it will be thin. Leave about an inch all the way around.
Speed up the cooling process by placing the pan in the refrigerator.
If you are in a place where it is very humid, you will likely have trouble getting your toffee to set. Toffee doesn't like humidity. Try it on a dry day!
Edited to add that some people have trouble w/ the chocolate separating from the toffee. This is kind of normal in almond roca since the toffee is so buttery. I've been told that one way to avoid that is to give the toffee a light dusting of cocoa powder before placing the chocolate bars on top. The powder apparently acts as an absorbent for the butter and allows the chocolate to adhere more easily to the toffee. I haven't tried it yet, but I'm told that it works!