Preheat the oven to 350 degrees F and grease a 9 x 13 inch cake pan (or a 10 x 15 inch jelly roll pan for a sheet cake).
Vegan buttermilk: In a measuring cup, measure 1 cup of soy milk, then add the apple cider vinegar and give it a little stir. Set aside to curdle.
Dry ingredients: In a medium bowl, whisk together the flour, baking powder and salt. Set aside.
In a large bowl, or the bowl of a stand mixer with a paddle attachment, beat together the peanut butter, sugar and oil until well combined, scraping the sides as needed.
To the bowl with the peanut butter, with the mixer on low speed, alternate adding the flour mixture and the vegan buttermilk, in 3 parts, ending with the buttermilk. The cake batter is thicker than most, and won't really pour like other cake batter.
Transfer to the prepared pan and spread out evenly. Bake for 28-32 minutes until a toothpick comes out clean in the middle. For a sheet cake (10x15 inch pan), it will only need 15-18 minutes in the oven.
Let the cake cool completely, then frost generously with peanut butter frosting and serve. Sprinkle with mini chocolate chips, if desired.