Peel and boil potatoes. Drain, add milk and mash. Set aside to cool.
Mix corn and green beans. Boil until near-done. It will cook fully in the oven later.
Brown ground beef in a deep pan. Mix beef bouillon with 1/3 cup of water and stir in the taco seasoning mix. Add to cooked ground beef and cook on medium heat for another 10 minutes, stirring occasionally.
Drain excess juice (meat mixture should be wet but not pooling in liquid), and add in the cooked vegetables. Mix well and put in a large casserole dish. Press the mixture down with a large spoon. Spread the mashed potatoes on top and flatten. Score the potatoes in a checkerboard style with a fork to make ridges. Put dabs of butter randomly on top.
Bake at 375°F for 40 minutes. If potatoes haven't browned, stick under the broiler for a few minutes but don't burn. Let cool slightly before serving.