In a large bowl, combine the first six ingredients; add 1¼ cups milk. Beat on low speed 30 seconds. Beat on medium 2 minutes. Transfer to a greased 4- or 5-quart slow cooker. Sprinkle cook-and-serve pudding mix over batter.
In a small saucepan, heat remaining milk, until bubbles form around sides of pan; gradually pour over the dry pudding mix.
Cook, covered, on high 3–4 hours or until a toothpick inserted in cake portion comes out with moist crumbs.
Turn off slow cooker. Sprinkle marshmallows, chocolate chips and pecans over cake; let stand, covered, 5 minutes or until marshmallows begin to melt. Serve warm. If desired, top with ice cream.