Preheat oven to 375°F. Sift flour and salt together in a bowl and set aside. Mix butter and date paste in a mixer on medium speed until light and fluffy (about 2 min.) Add egg and vanilla and mix until incorporated. While mixer runs on lowest speed, gradually add flour and mix just until mixed in and dough ball forms. Scrape down sides of bowl, cover with plastic wrap and refrigerate for at least one hour. Using a small cookie scoop or a tablespoon to measure, roll dough into balls. Roll the balls in chopped walnuts and place on parchment lined baking sheets at least 2 inches apart. Using your thumb, make an indentation in the middle of the cookies. Refrigerate for 20 minutes. Bake for 15-20 minutes or until very lightly brown. While still warm, gently press a chocolate kiss into the indentation. Allow cookies to cool completely on a wire rack. Store in an airtight container.