In a medium bowl, add softened butter or margarine, masa harina flour and sugar. Beat with a mixer until light and fluffy, about 1 minute.
Add 1 cup corn kernels and water to a blender. Blend until smooth.
Add corn mixture from the blender to the bowl with masa mixture. Stir well to combine.
Add remaining corn, cornmeal, baking powder, salt and milk. Mix well to fully combine.
Pour mixture into an 8-inch baking dish. Tightly cover with plastic wrap. Make sure to seal tightly.
Fill a medium saucepan with water.
Place saucepan over high heat. Bring to a rapid boil.
When water is fully boiling, place a screen or cooking rack across the pan.
Set sealed baking dish on top of the screen above the boiling water.
Making sure to keep the baking dish tightly wrapped, steam for 50 minutes to 1 hour. (Be sure to check the water level often, so as not to boil dry.)
After 50 minutes, check for doneness by inserting a toothpick into the center of the dough. When it comes out clean, the tamalito is done. Cook longer as needed.