Put pecans on a rimmed baking sheet and cook, stirring occasionally, until toasted, about 8 minutes. When cool enough to handle, chop roughly. Divide into 2 batches; chop 1 batch finely. Set both batches aside.
Put sugar, butter, salt, and 3/4 cup water in a 4-qt. saucepan over medium heat (see Notes). When butter and sugar are melted, increase heat to medium-high and cook, stirring occasionally, until mixture is deep golden brown and measures 310 on a candy thermometer, about 20 minutes.
Remove from heat and carefully stir in vanilla (mixture will bubble up) and finely chopped pecans.
Pour into a 10- by 15-in. rimmed baking sheet.
Let toffee cool until set, at least 30 minutes. (For even pieces, you can "score" the toffee by cutting it with a sharp knife after it has set for about 10 minutes, scoring into 5 strips lengthwise and 8 strips crosswise to yield 40 pieces. Wipe knife clean with warm water after each slice for easier cutting.)
Chop chocolate and put in a medium metal bowl. Fill an 8- or 10-in. frying pan with 1/2 in. of water and bring to a boil. Take off heat and put bowl of chocolate in the water.
Let sit about 5 minutes. Stir chocolate until melted.
Pour over toffee; with a knife or offset spatula, spread evenly.
Sprinkle chocolate with roughly chopped pecans.
Let sit 20 minutes, or until chocolate is cool but still a bit soft.
Sprinkle with fleur de sel. Chill until set, about 1 hour.
To remove, gently twist pan to release toffee, then chop or break into chunks. Store in an airtight container.
