Preheat oven to 350°F. Grease and flour a 9x13-inch baking pan.
In a large mixing bowl, cream the butter and sugar until light and fluffy. Beat in eggs, one at a time.
In a separate bowl, whisk together the flour, cocoa powder, baking soda, and salt.
Add the dry ingredients to the butter mixture alternately with the milk, mixing until smooth.
Pour the batter into the prepared baking pan and bake for 35-40 minutes, or until a toothpick inserted in the center comes out clean.
Allow the cake to cool for 10 minutes. Using the handle of a wooden spoon, poke holes evenly across the top of the warm cake.
For the filling, combine the sweetened condensed milk and Andes Creme de Menthe baking chips in a microwave-safe bowl. Microwave in 30-second intervals, stirring between each interval, until melted and smooth.
Slowly pour the mint filling over the cake, allowing it to soak into the holes. Let the cake cool completely.
For homemade mint whipped cream, beat the heavy whipping cream until soft peaks form. Add the powdered sugar and mint extract, then continue beating until stiff peaks form. Add green food coloring if desired.
For a shortcut topping, use 4 cups of whipped topping such as Cool Whip and fold in the powdered sugar, mint extract, and optional food coloring until combined.
Spread the topping evenly over the cooled cake.
Garnish with mini chocolate chips, Andes mint pieces, or sprinkles.
Refrigerate for at least 4 hours or overnight before serving.