Preheat oven to 350°F.

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°F 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°F; 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.