Step 1: Preheat your oven to 350°F (175°C). Grease a 9x13 inch baking dish.
Step 2: In a small saucepan over low heat, combine the peanut butter, milk, and granulated sugar. Stir until smooth and warmed through. Remove from heat.
Step 3: Pour the peanut butter mixture into the prepared baking dish, spreading it evenly across the bottom.
Step 4: Sprinkle the dry chocolate cake mix evenly over the peanut butter layer. Do NOT stir.
Step 5: Evenly distribute 1 cup of the mini peanut butter cups (and chocolate chips, if using) over the cake mix layer.
Step 6: Drizzle the melted butter over the entire surface of the cake mix and peanut butter cups. Ensure the dry cake mix is as covered as possible by the butter.
Step 7: Bake for 45-50 minutes, or until the cake is bubbly around the edges and the top is lightly golden. The center will still be gooey.
Step 8: Remove from oven and immediately sprinkle the remaining ½ cup of mini peanut butter cups over the hot cake. They will melt slightly.
Step 9: Let the cake cool for 15-20 minutes before serving warm with a scoop of vanilla ice cream, if desired.