Using an electric mixer (stand or handheld), cream together butter, sugar, vanilla and salt until smooth, about 3 min. Scrape down sides of bowl, add egg and mix well. Scrape down sides of bowl again, slowly add flour and half of the chopped cashews. On a sheet of plastic wrap, shape dough into a 1-in. (2.5 cm) thick rectangle, wrap and refrigerate until chilled, about 20 min.
Preheat oven to 350°F (180°C). Dust a piece of parchment paper with flour, place dough in centre, dust top of dough with flour and cover with another sheet of parchment paper. Roll dough to about a 1/4-in. (5 mm) thickness, dusting with more flour if necessary. Remove top sheet of parchment (use it to line your baking sheet), cut cookies into 1 x 3-in. (2.5 x 8 cm) bars and place on prepared baking sheet, spaced about 1 in. (2.5 cm) apart. Bake on middle rack of oven until set, but not golden, about 12 min. Transfer cookies to a cooling rack and let cool completely.
Meanwhile, in a small saucepan set over low heat, melt caramels with cream, stirring occasionally. Let mixture cool to room temperature, then drizzle over cookies. Sprinkle salt and remaining cashews overtop and let cool before serving.