Preheat oven to 350° and line a rimmed baking sheet with parchment paper.
Using a stand mixer and paddle attachment, blend almond butter with coconut sugar on high speed 1 minute. Add vanilla and beat 1 minute more.
In another bowl, whisk oat flour with oats, baking powder, and salt.
On medium speed, blend half of almond milk into almond-sugar mixture, then blend in half of dry ingredients. Repeat, taking care to mix just until blended; then stir in chocolate chips. Dough will be thick and crumbly.
Turn dough onto baking sheet and gather into a mound. Press out into an even rectangle 1/2 in. thick and about 7 by 9 1/2 in. (the thickness is more important than the size).
Bake until edges just begin to darken and crack and a toothpick inserted into center comes out with just a few tiny crumbs, 15 to 20 minutes. Let cool on pan until firm, then lift to a rack to cool completely.
Slide parchment with cookie rectangle onto a cutting board. Cut into quarters or thirds lengthwise, then into thirds crosswise. If you like, wrap each bar in parchment and tie with kitchen twine.
MAKE AHEAD up to 2 weeks, chilled.