Preheat oven to 350°F (177°C). Line an 8-inch square baking pan with parchment paper or spray with nonstick spray. Set aside.

With an electric or stand mixer fitted with a paddle attachment, mix the brown sugar and peanut butter on medium speed until light in color and fluffy, about 3 minutes. Mix in the vanilla, scraping down the sides as needed.

On medium speed, add in the flour, oats, salt, and baking soda. The dough will be very thick and clumpy, but it will come together. Slowly add the milk in a steady stream, mixing until a dough forms. Fold in chocolate chips and raisins. Use your hands to make sure everything is combined, if needed.

Once dough is completely combined, press it lightly into the prepared baking dish. Bake for about 17-20 minutes until the bars are lightly golden and set. Allow to cool completely and cut into squares. I keep mine in the refrigerator in a covered container. Bars will be good stored up to 2 weeks.