Preheat oven to 425 degrees. Line a baking sheet with parchment paper.
Boil potatoes: Add potatoes to a large pot. Cover with cold water (covering the potatoes by about 1 - 2 inches) and season with 1 Tbsp of salt. Bring to a boil over medium-high heat, then reduce heat to medium-low, cover pot with lid and allow to simmer until very tender, about 15 - 20 minutes (they should nearly fall apart when pierced). Drain potatoes well.
Saute garlic in butter: Meanwhile, dice 4 Tbsp of the butter into 1 Tbsp pieces and add to a small saucepan. Melt over medium heat then add garlic and saute until lightly golden and softened, about 1 - 2 minutes.
Stir in half and half: Remove from heat, stir in 3 Tbsp of the half and half then pour mixture into a small bowl (so the garlic doesn't burn) and set aside.
Mash potatoes: Pour drained potatoes into a large mixing bowl. Mash well with a potato masher until there are no longer any lumps (or press through a potato ricer).
Stir in cheese and butter mixture: Stir in parmesan cheese and warm butter/garlic mixture with a wooden spoon and season with salt and pepper to taste.
Blend in yolks: Let mixture cool a few minutes then quickly stir in egg yolks one at a time with a wooden spoon. At this point if mixture is very thick you can add remaining 1 Tbsp half and half if needed.
Pipe into domes: Transfer mixture to a large piping fitted with a large star tip that's about 3/4 inch wide at tip (or you can just spoon mounds onto baking sheet vs. piping). Pipe into 12 mounds spaced evenly apart, about 2 1/2-inches wide by 2 1/2-inches tall. Gently brush with remaining 2 Tbsp butter, melted.
Bake: Bake in preheated oven until golden brown, about 16 - 19 minutes. Serve immediately garnished with parsley and parmesan if desired.