Make the Cookies: Preheat the oven to 375°. Spread the nuts in a pie plate and toast for 8 minutes; let cool, then chop.
In a bowl, mix the flour, baking soda and salt. In the bowl of a standing mixer fitted with the paddle (or using a handheld mixer), cream the butter with the sugars and vanilla at medium speed, about 1 minute. Beat in the egg. With the mixer at low speed, beat in the dry ingredients. Beat in the walnuts and chocolate chips. Spoon level tablespoons of the dough onto 2 ungreased baking sheets, about 2 inches apart. Refrigerate for 30 minutes, until firm.
Make the Ganache: Put the chocolate in a bowl. In a saucepan, bring the cream and corn syrup to a boil; pour over the chocolate and let stand for 1 minute. Whisk until smooth. Whisk in the crème fraîche. Refrigerate the ganache, stirring occasionally, until thick and spreadable, 1 hour.
Bake the cookies for 12 minutes, until golden; let cool on the sheets for 2 minutes, then transfer to a rack to cool completely.
Sandwich the chocolate-chip cookies with the ganache and serve.