Prepare cake batter according to package directions, adding 1 1/2 tsp mint extract to the batter. Pour all but 1 cup of the batter into a well greased 9x13" pan.
Add 3-4 drops food coloring to reserved batter. Drop by spoonfuls on top of white batter. Swirl with a knife. Bake as instructed on box. Cool completely.
While cake is cooling, combine sweetened condensed milk, chocolate chips, and milk in a glass bowl. Melt in the microwave, stirring every 30 seconds till smooth. Let cool. When cake is cooled, spread chocolate fudge over the top.
Whip cream till soft peaks form. Add powdered sugar, 1/2 tsp mint extract, and food coloring. Beat till well mixed, then spread over fudge layer. Garnish with Andes Mints if desired.