Ice Cream: Combine milk and cream in heavy saucepan. Whisk in combined sugar, dextrose, dry milk and stabilizer. Cook on medium heat until mixture reaches 165°F, stirring frequently. Whisk in vanilla.
Blend mixture, in small batches, in blender 30 seconds. Pour into large bowl. Chill bowl over ice water bath until mixture is cold. Refrigerate overnight.
Place a sheet pan in freezer. Freeze cream mixture in ice cream maker according to manufacturer's directions. When firm, stir in OREO Pieces. Cover bottom of frozen sheet pan with parchment paper. Spread ice cream evenly to depth of pan. Freeze until ready to use.
Brownies: Whisk eggs, oil and corn syrup in large bowl until blended. Sift dry ingredients, except cookie pieces, in separate bowl then add to egg mixture. Mix until blended. Divide mixture between 2 parchment paper-lined half sheet pans. Top evenly with cookie pieces.
Bake in 350°F conventional oven 20 min. Cool completely in pans on wire rack. Refrigerate for ease in cutting.
Trim 1 long end and 1 short end of each pan of brownies. Cut each pan into 25 (3x2-inch) rectangles. Trim 1 long end and 1 short end of ice cream slab. Cut ice cream into 25 (3x2-inch) rectangles.
For each Ice Cream Sandwich: Melt dipping chocolate. Sandwich 1 ice cream rectangle between 2 brownies. Dip 1 long side of sandwich into chocolate to 1/2-inch depth. Allow chocolate to harden. Press chopped pretzels and OREO Medium Pieces onto the 3 "bare" sides of each sandwich. Freeze until ready to serve.