Preheat oven to 350°. Combine first 3 ingredients in a food processor; process until finely ground. Lightly spoon flour into dry measuring cups; level with a knife. Combine oat mixture, flour, baking powder, salt, and sugar in a large bowl, stirring with a whisk. Combine milk, vanilla, and eggs in a small bowl; stir with a whisk until well blended. Add egg mixture to flour mixture, stirring just until moist. Turn dough out onto a floured surface; knead lightly seven times with floured hands. Shape dough into a 15 x 3–inch log on a parchment paper–lined baking sheet; pat to 1-inch thickness. Bake at 350° for 30 minutes. Remove roll from baking sheet; cool on a wire rack. Cut roll diagonally into 18 (1/2-inch) slices. Place, cut sides down, on baking sheet. Reduce oven temperature to 325°; bake 18 minutes. Turn cookies over; bake an additional 18 minutes (cookies will be slightly soft in center but will harden as they cool). Remove from baking sheet; cool completely on wire rack. Place chips in a small microwave-safe bowl; microwave at HIGH 30 seconds or until almost melted, stirring until smooth; spread evenly over tops of biscotti.