Add macaroni to a pot of salted boiling water, cook until just short of al dente (short the cooking time by one minute). Drain. Transfer macaroni back into pot.
While macaroni cooks, spray a saucepan with nonstick cooking spray and turn on to medium low heat. Add ¾ cups milk, sharp cheddar and parmesan cheese. Stir constantly, for about 8 to 10 minutes, or until it looks very thick and a bit stringy (it won’t look quite saucy yet).
Add green onions, cheese mixture and ½ cup milk to macaroni. Turn heat onto medium high and stir for a minute or two, until cheese sauce starts to melt into a creamy consistency. Season to taste with salt and pepper. Remove from heat.
Spray two muffin tins with nonstick cooking spray. Pack macaroni and cheese into muffin cups, leaving some room at the top. Sprinkle a layer of breadcrumbs over each cup, press down into macaroni and cheese. Then add a sprinkling of parmesan cheese to each cup.
Bake at 375F for 30 minutes. Remove from oven and let cool completely, about 30 minutes, before removing from muffin tin. They must be completely cooled before removing.