In a large bowl, cream butter and sugars until light and fluffy, 5-7 minutes. Beat in egg and vanilla. In another bowl, whisk flour, pecans, baking powder, salt and cinnamon; gradually beat into creamed mixture. Dough will be soft. Form dough into a disk; cover and refrigerate 1 hour or until chilled.
Preheat oven to 350°. On a floured surface, roll dough to 1/4-in. thickness. Cut with a floured 2-in. round cookie cutter; reroll dough scraps as needed. Place 2 in. apart on parchment-lined baking sheets.
Bake until edges begin to lightly brown, 10-12 minutes. Remove from pans to wire racks to cool completely.
For buttercream, in a large bowl, beat butter, cream and vanilla until creamy. Beat in confectioners’ sugar alternately with dulce de leche until smooth. Sprinkle cookies with additional confectioners' sugar. Pipe buttercream onto cookies; sprinkle with pecans. Store, covered, in refrigerator.