In a 3-quart saucepan over medium-low heat, melt the butter. Add the sugar, brown sugar, 1/3 cup water, Karo and salt. Stir with a wooden spoon until the sugar dissolves. Raise the heat to medium. Cook at a gentle boil for 10 minutes, stirring often, until temperature reaches 290°. Add almonds and stir until thermometer returns to 290°.
Remove from heat. Immediately pour the mixture onto the prepared sheet and spread. Sprinkle 6 ounces of the chocolate over the toffee. Let stand for 2 minutes. Using a small offset metal spatula, spread the chocolate over the toffee. Sprinkle with 1 cup of finely chopped almonds. Press gently Refrigerate uncovered until firm, about 2 hours.
Microwave remaining chocolate until melted.
Invert toffee onto a waxed paper–lined baking sheet. Spread with melted chocolate and sprinkle with remaining almonds. Press gently. Refrigerate uncovered for 2 hours or until set.
Break into pieces. Store in an airtight container in the refrigerator.