Preheat oven to 350 degrees and oil a large baking sheet.
In a large baking pan spread almonds evenly and toast in the middle of the oven, stirring nuts halfway through toasting until golden, about 10 minutes.
In a 3-quart heavy saucepan bring butter, sugar, water, lemon juice, vanilla, and salt to a boil over moderate heat, stirring with a wooden spoon. Boil mixture, without stirring, swirling pan occasionally, until deep golden, about 12 minutes.
Remove pan from heat and stir in two-thirds of the almonds. Immediately pour toffee onto baking sheet and with an offset spatula spread into a thin layer. Carefully transfer baking sheet (it will be very hot) to a rack and cool toffee.
Chop chocolate. In a double boiler or small metal bowl set over a saucepan of barely simmering water melt chocolate, stirring until smooth. Pour chocolate over cooled toffee and evenly spread with offset spatula. Sprinkle toffee with remaining almonds. Chill toffee, uncovered, until firm, about 1 hour.
Break toffee into 2-inch pieces of "bark." Bark keeps, layered between sheets of wax paper in an airtight container at cool room temperature or chilled, 1 week.