Slice the tip off the garlic bulb to expose the cloves and drizzle with oil. Wrap in foil then place in the oven at 180C/356F for 50-60mins, or until very soft, golden and caramelised. Leave to cool for a few mins, then squeeze out as much as you can, making sure the outer peel doesn't mix in. Sprinkle over a pinch of salt then mash with a fork to smooth out any lumpy parts. Increase oven temp to 200C/400F.
Meanwhile, peel and dice the potatoes into large bite-sized pieces. Add to a large pot of cold water with 2 tsp salt. Bring to a boil and cook until the potatoes are fork tender (they should just about break apart when stabbed). Drain and leave in the colander for 5 minutes to steam dry, shaking once halfway.
Add the potatoes to the pot and mash with 3 tbsp / 45g butter alongside the cream. Stir in the parmesan and roasted garlic, mashing a little more to smooth out any lumps if needed. Generously season with salt and pepper (to taste). If the mash is still aggressively steaming just leave it to cool for a few mins, then one by one firmly stir in the egg yolks until well blended.
Spoon the mash into a piping bag fitted with a large star tip (you'll need to work in batches). Pipe the mash into swirly dome shapes on a large baking tray with baking paper. They should be around 2.5-3"/6.5-7.5cm wide/high and you should get 10 from the mix.
Carefully drip butter over the tops (it should swirl down and coat the potatoes) then bake in the oven for 15-20mins, or until golden brown. Be vigilant as they can char quickly and turn the tray around if one side is cooking more than the other. Lower temp slightly if they start charring quickly.
Serve with parsley and more parmesan and then tuck in and enjoy!