Preheat the oven to 350°F (175°C). Spray a glass 9x13-inch pan with cooking spray.

Combine brownie mix, oil, water, and eggs in a large bowl. Stir 50 strokes with a spoon. Spread into the prepared pan.

Bake in the preheated oven until a toothpick inserted into the center comes out clean, 28 to 31 minutes. Remove from the oven and allow to cool slightly, about 10 minutes.

Break up warm brownies into a large bowl. Roughly chop mint thins and add to the bowl; stir until well combined, allowing the brownies' heat to slightly melt the chocolate.

Form mixture into 1 1/2-inch balls, pressing firmly so they hold together. Chill in the refrigerator for 20 minutes.

Combine chocolate chips and oil in a small microwave-safe bowl; microwave in 30-second intervals, stirring after each one, until melted and smooth.

Dip each brownie ball into the melted chocolate and turn with a fork to coat. Lift and allow excess chocolate to drip off. Place dipped truffles onto a tray lined with waxed or parchment paper. Top with sprinkles, if desired.