Prepare the potatoes by boiling them until tender, about 15-20 minutes.
While the potatoes cook, heat 2 tablespoons of butter in a skillet over medium-high heat.
Add minced garlic to the skillet and sauté for about 30 seconds, until fragrant.
Add the cubed sirloin steak to the skillet, and season with salt and pepper.
Cook the steak for about 6-8 minutes, stirring occasionally to achieve even browning.
Add the remaining butter and parsley, stirring to combine before removing from heat.
Drain the potatoes and return them to the pot. Add the heavy cream and milk, and mash until smooth.
Season the mashed potatoes with salt and pepper, to taste.
Serve the garlic butter steak bites over the creamy mash, garnished with additional parsley, if desired.