Heat oven to 350ºF. Combine brown sugar and butter in bowl. Beat at medium speed, scraping bowl often, until creamy. Add bananas, eggs and vanilla. Continue beating until well mixed. Add flour, oats, baking soda and salt; beat at low speed until well mixed.
Drop dough by 1/4 cupfuls, 3 inches apart, onto ungreased cookie sheets. Flatten dough to 3-inch diameter with bottom of glass dipped in sugar. Bake 14-19 minutes or until edges are lightly browned. Let stand 2 minutes; remove from cookie sheets. Cool completely.
Chill baking sheet 5 minutes in freezer.
Scoop 12 (about 1/4 cup) ice cream portions onto chilled baking sheet. Return to freezer; freeze until ready to assemble sandwiches.
Melt chocolate chips and shortening in small saucepan over medium-low heat, stirring constantly, 2-3 minutes or until chocolate is melted and smooth.
Thaw ice cream portions 4-5 minutes at room temperature. Place 1 portion onto bottom-side of 1 cookie; top with another cookie, bottom-side down. Press cookies together gently. Smooth ice cream with knife, if necessary.
Spread 1 tablespoon chocolate over top of each sandwich cookie; sprinkle with 1 teaspoon walnuts. Place onto baking sheet; freeze immediately at least 2 hours or until set.
Wrap each frozen ice cream sandwich in plastic food wrap; place into large resealable freezer bag. Store frozen.
When chocolate has set, place ice cream sandwiches into resealable small plastic freezer bags. Store in freezer.