Place almonds in a baking pan. Bake in a 350° regular or convection oven, shaking pan occasionally, until golden beneath skins, 10 to 12 minutes. When cool enough to handle, finely chop.
In a 5- to 6-quart pan over medium-low heat, stir sugar, butter, corn syrup, salt, and 3/4 cup water until butter is melted and sugar is dissolved. Increase heat to medium-high and cook, stirring occasionally, until mixture is deep golden brown (300° on a candy thermometer; see notes), 10 to 15 minutes. Remove from heat and carefully stir in vanilla and half the almonds (mixture may bubble up). Immediately pour into a 10- by 15-inch baking pan with 1-inch-tall sides. Let toffee cool at room temperature until set, at least 30 minutes.
Meanwhile, place chocolate in the top of a double boiler or in a heatproof bowl. Bring a few inches of water to a simmer in bottom of double boiler or a pan that the bowl can nest in; remove pan from heat. Place chocolate over water and let stand, stirring occasionally, until melted and smooth, about 10 minutes.
Pour chocolate over cooled toffee; with a knife or an offset spatula, spread level. Sprinkle remaining almonds evenly over chocolate. Let stand at room temperature until chocolate is set, at least 1 hour (or chill about 30 minutes).
To remove, gently twist pan to release toffee, then chop or break into chunks. Store airtight at room temperature for up to 2 days, or chill airtight up to 1 month.
Before you begin, read the recipe all the way through and assemble all the tools and ingredients you will need. Many candy recipes require that you act quickly once the sugar syrup reaches the desired temperature. Use care when working with hot sugar syrup, as it can cause severe burns.
Choose the right pans. Heavy-bottomed stainless steel pans are best for cooking sugar mixtures. Thin, lightweight pans tend to conduct heat--and cook sugar syrup--unevenly.
Use a candy thermometer when called for. They meausre temperatures up to 400°. You'll find them in the kitchen-gadget section of many supermarkets, priced between $10 and $
Submerge thte bottom of the thermometer completely in the sugar syrup to get an accurate reading. Using a narrow pan with tall sides makes the mixture deeper, but, if necessary, you can gently tilt a shallower pan to submerge the thermometer bottom.
Melt chocolate gently for best results. If chocolate gets too hot, it may not set properly and will develop "bloom" (white streaks) on the surface when stored. Stirring chopped chocolate in a pan or bowl over hot, not simmering, water maintains an even, low temperature, resulting in glossy, firmly set chocolate.
Nutritional analysis per ounce.