Butter a 12" x 18" jelly roll pan. Set on a heat resistant surface or trivets.
Gather all ingredients and tools BEFORE beginning to cook: butter, water, sugar, almonds, candy thermometer, pastry brush and small bowl of water, wooden spoon.
In a large 4 QT pot, combine butter and water, and turn burner to high. Once butter is melted, add sugar and stir constantly until it is completely dissolved. With your pastry brush and bowl of water, wash down the sides of the pot to dissolve any sugar crystals. (Dip your brush into the water and wash down a section, then repeat until you've made it all the way around.)
Turn your burner down to medium low. (On my stove, high is 7, 4 is medium, and I turn it down to 3 for this stage.) Add almonds, clip candy thermometer onto the side of the pan, and stir constantly and VERY SLOWLY until the mixture reaches 290 F/145 C.
Remove from heat immediately and pour into prepared pan. (Tip the pan back and forth to spread it out evenly, if needed.)
If covering with chocolate, let the toffee sit at room temperature for about 45 minutes until just slightly warm. Melt the chocolate chips with the shortening in a double boiler on the stove or in the microwave (start with 2 minutes on 50% power, stir, then go another minute or so until completely melted). Place a paper towel on top of the toffee to soak up any oil that may be present (if left, the chocolate may have trouble sticking). Spread the chocolate on top of the toffee as evenly as possible.
Let toffee sit uncovered overnight at room temperature, then break into small pieces. Store in an airtight container.