Preheat oven to 350 F. Place almonds on a baking sheet and bake them for 8-10 minutes, or until golden brown and fragrant. Set aside to cool. Using a food processor or chef's knife, finely chop the almonds.
Line an 8x10 inch rimmed baking sheet with aluminum foil, leaving at least 1-inch to overlap the top of the pan on all sides. Lightly coat the surface of the foil with butter or nonstick cooking spray. Attach a candy thermometer to a heavy bottomed sauce pan. Place the candy thermometer at least 1/2 inch off the bottom of the pan.
In a medium-sized, heavy saucepan combine the brown sugar, water, butter, and corn syrup. Over medium heat, bring the sugar mixture to a boil. With a heatproof pastry brush that has been dipped in warm water, wash down the sides of the saucepan to remove any sugar crystals that may have formed. Cook, without stirring, until the mixture reaches 285 F (soft-crack stage) on a candy thermometer. Immediately remove the saucepan from the heat and add the baking soda and vanilla extract and stir to combine.
Immediately pour this mixture into the baking sheet. Then place the chopped chocolate over the hot toffee. After a few minutes the chocolate will be soft enough to spread with an offset spatula in an even layer over the toffee. Sprinkle chopped almonds over the melted chocolate. Cool completely and then cut into desired shapes using a sharp knife.