For the cake: Preheat the oven to 350 degrees F, and grease a 10 x 15 baking pan. (See notes)
In the bowl of a stand mixer or another large bowl, whisk together the dry ingredients. Add the remaining cake ingredients, except the mini chips, and beat until well blended. Use a spatula to make sure the dry ingredients on the bottom of the bowl are thoroughly incorporated. Fold in the chocolate chips.
Spread mixture in the prepared pan, and bake for 20-25 minutes or until the cake is just cooked through, being careful not to over-bake.
Allow the cake to cool completely and then frost. (If you prefer a thinner layer of frosting, you may have a small portion leftover.) Sprinkle the top with mini chocolate chips, if desired.
For the Foolproof Peanut Butter Frosting: In a mixing bowl, beat the peanut butter and butter until creamy. Add about half of the powdered sugar and blend well. Add the milk and vanilla. Blend again.
Beat in the remaining powdered sugar until the mixture is fluffy and smooth.
Frosting may be prepared several days in advance and stored, covered, in the refrigerator. Bring to room temperature and stir well prior to using.