Lightly grease a 6-quart oval slow cooker.

With an electric mixer, beat cake mix, water, eggs, 1/2 cup peanut butter and butter on medium speed for 2-3 minutes, or until smooth.

Remove 2/3 cup of batter to a medium bowl and stir in 1/2 cup chocolate syrup.

Spoon 1/2 of the peanut butter batter into the slow cooker, followed by all of the chocolate batter. Top with remaining peanut butter batter and swirl batters together with a knife.

Cover and cook on high heat for about 2 hours (check at 1 hour 45 minutes) or until toothpick inserted in center of cake comes out mostly clean. Turn off slow cooker, uncover, and let stand 10-15 minutes more.

Meanwhile, whisk together 3 tablespoons peanut butter and 2 tablespoons milk until smooth. And powdered sugar and mix until smooth. Add additional milk as needed, until glaze reaches desired consistency.

Spread peanut butter glaze over warm cake in slow cooker. Drizzle with chocolate syrup and sprinkle with miniature peanut butter cups. Serve warm with ice cream, if desired.