Preheat & Prep Pan: Preheat oven to 350°F (177°C). Line an 8 inch square baking pan with parchment paper, leaving overhang, or spray with nonstick spray. Set aside.
Cream Peanut Butter & Sugar: In a large bowl using a hand mixer or a stand mixer with paddle attachment, beat the brown sugar and peanut butter together on medium speed until light in color and fluffy, about 3 minutes. Mix in the vanilla, scraping down the sides as needed.
Add Dry Ingredients: On medium speed, add the flour, oats, baking soda, and salt. The dough will be very thick and clumpy.
Add Milk: With the mixer running, slowly pour in the milk in a steady stream. Mix until a cohesive dough forms.
Fold in Mix Ins: Use a spatula or your clean hands to fold in the chocolate chips and raisins until evenly combined.
Press into Pan: Transfer the dough to the prepared pan. Press it firmly and evenly into the pan.
Bake: Bake for 17-20 minutes, or until the edges are lightly golden and the top is set.
Cool & Slice: Allow bars to cool completely in the pan on a wire rack. Once cool, use the parchment to lift them out and cut into squares.
Optional Drizzle: For an extra treat, melt 3 tablespoons of chocolate chips with 1 tablespoon of peanut butter and drizzle over the cooled bars.