Toast 1/2 cup of the almonds in a dry skillet. Set aside to cool.
Line a large rimmed sheet pan or jellyroll pan with parchment paper and set next to stove.
Place butter in a heavy bottomed, medium sized (3 qt) saucepan and melt over medium heat. When butter is mostly melted, stir in sugar, warm water and salt.
Set a deep fry thermometer in the pan, being careful that bulb is not touching bottom, and cook without stirring over medium heat until mixture reaches 240 degrees F.
At 240 degrees, add the remaining 1 cup sliced almonds to sugar mixture and begin stirring constantly, keeping heat at medium, until mixture reaches 295 degrees F. If temperature is not rising at a slow and steady rate, raise heat a tiny bit until mercury starts rising
When mixture reaches 290 (make sure it's no less than 290 and no more than 295 degrees F), immediately remove from heat and stir in baking soda. It will bubble up and lighten a bit and seem almost fluffy. Pour into parchment lined pan. The mixture should be kind of a liquid blob at this point and if you are on track, the saucepan will be clean when you dump the blob of candy onto the parchment. Mixture will begin to firm, and butter may pool and separate. Do not pour off any excess butter. Do not panic, because if you cooked the toffee to between 290 and 295, your candy should set. Sometimes it just takes longer.
While candy is still very hot and in the process of setting, scatter chopped chocolate across top of hot and let chocolate melt into and over the candy as candy firms. Using back of a spoon, spread melted chocolate evenly over candy. Crush your toasted almonds and sprinkle over melted chocolate. Let candy cool for a 1/2 hour or more at room temperature. Transfer to refrigerator and chill for about 1 hour to firm chocolate.
When chocolate is set is set, lift candy from pan by grasping parchment. Break candy into large chunks.