Preheat oven to 400 degrees.
Grate sweet potatoes and set aside.
Melt butter.
Cream butter and agave syrup together.
Mix in eggs, pumpkin pie spice, salt, half & half and milk with butter and agave syrup. Beat with a whisk.
Fold sweet potatoes into the milk mixture, evenly incorporating.
Butter baking dish with a pat of butter.
Pour sweet potato mixture into the dish, sprinkle with sugar and bake for 1 hour.
Serve immediately.