Place the peeled and chunked russet potatoes in a large pot and cover with cold water by about 1 inch.
Add a pinch of salt to the pot, bring the water to a boil over medium-high heat, then reduce to a gentle simmer.
Cook the potatoes for 15–20 minutes, or until they are very tender when pierced with a fork.
Drain the potatoes well and return them to the warm pot, letting excess moisture evaporate for a minute.
Add the unsalted butter for the potatoes, warm milk, garlic powder, salt for the potatoes, and black pepper for the potatoes.
Mash the potatoes until smooth and creamy, adjusting the seasoning to taste, then cover the pot to keep warm.
Pat the shrimp dry with paper towels, then season with half of the salt and half of the black pepper.
Heat the olive oil and 1 tablespoon of the unsalted butter in a large skillet over medium-high heat.
Add the shrimp in a single layer and cook for 1–2 minutes per side, just until pink and opaque, then transfer them to a plate and set aside.
Reduce the heat to medium, add the remaining unsalted butter to the skillet, and let it melt.
Stir in the minced garlic, paprika, and crushed red pepper flakes (if using), cooking for about 30 seconds until fragrant, stirring constantly to avoid burning.
Pour in the chicken broth, scraping up any browned bits from the bottom of the skillet, and let it simmer for 2–3 minutes to reduce slightly.
Lower the heat and stir in the heavy cream, then sprinkle in the remaining salt and black pepper, stirring to combine.
Add the grated Parmesan cheese and stir gently until the sauce is smooth and slightly thickened.
Return the cooked shrimp and any juices on the plate to the skillet, tossing them in the creamy garlic sauce for 1–2 minutes to warm through.
Stir in most of the chopped fresh parsley, reserving a little for garnish, and taste the sauce, adjusting seasoning if needed.
To serve, spoon a generous portion of mashed potatoes onto each plate and top with the creamy garlic shrimp and sauce.
Garnish with the remaining fresh parsley and serve immediately while hot.