Heat oven to 350°F.
Mix together flour, pecans and salt in medium-size bowl.
Beat together butter, granulated and powdered sugars in second medium-size bowl until light and fluffy, about 3 minutes. Beat in eggs, one at a time, until well blended. Beat in vanilla. On low speed, beat in flour mixture just until dough forms. Cover and refrigerate for at least 1 hour for dough to firm up.
Using 2 teaspoons dough for each thumbprint, roll dough between palms into balls. Transfer to ungreased baking sheet, spacing 1-1/2 inches apart. With wooden spoon handle or finger, make slight indentation in top of each cookie.
Bake in 350°F oven for 10 to 11 minutes or until lightly browned. Let cookies cool slightly on baking sheets on wire racks. Then transfer the cookies to wire racks to cool completely.
Meanwhile, while cookies are cooling, heat heavy cream in small saucepan just until small bubbles appear around edge. Pour over chocolate in small bowl, stirring to melt the chocolate completely. Let cool slightly.
Spoon slightly cooled chocolate into the indentations in the cookies. If using the pecans, press a pecan half into the chocolate on each cookie. The let stand at room temperature until the chocolate sets, or refrigerate the cookies for a quick set.