1 cup (8 ounces) granulated sugar
16 tablespoons (2 sticks) butter
1/4 cup (2 ounces) water
1/4 teaspoon salt
1 tablespoon vanilla extract
1 cup chocolate, melted
1 1/2 cups (6 ounces) chopped pecans
Lightly butter a 10X15-inch sheet pan or line it with parchment paper.
In a 2-quart heavy saucepan, melt the butter. Add the sugar, water, salt and vanilla. Over medium to medium-high heat, bring the mixture to a boil, stirring constantly with a heat-resistant rubber spatula. Clip a candy thermometer to the side of the pan and continue stirring while cooking until the thermometer reaches 300 degrees F and the buttercrunch is light golden brown.
Pour the toffee onto the prepared pan and spread quickly into a thin even layer with an offset spatula. Allow the mixture to cool completely to room temperature. If needed, blot the buttercrunch with a paper towel to remove any droplets of butter.
Spread half of the melted chocolate on the top of the buttercrunch and top with half of the chopped pecans, pressing the pecans lightly into the chocolate to adhere. Allow the chocolate to set at room temperature. When set, carefully flip the buttercrunch over onto the back of another sheet pan lined with parchment paper or a silicone liner. Spread the second half of the buttercrunch with the rest of the chocolate Sprinkle the remaining pecans over the top, pressing them lightly into the chocolate.
Allow the chocolate to set fully. Break the buttercrunch into desired pieces. Store at room temperature in an airtight container.