Fill a small saucepan with water and bring to a boil and let the water simmer, choose a heat safe bowl that would sit nicely over the sauce pan and add the white chocolate to it.
The bowl should not come into contact with the water in the saucepan as this can cause over heating. Remove the bowl from the saucepan while there are still a couple of small lumps of unmelted white chocolate, stir until completely melted and smooth. If there are a few lumps that didn't melt just put the bowl back over the saucepan of hot water for a minute or two and stir again. Set chocolate aside to cool to almost room temperature.
Preheat oven to 340F.
In the bow of an electric mixer fitted with the whisk attachment (or using a hand mixer) whisk the butter until smooth and fluffy, about 1 minute. Add the cooled down white chocolate to the whipped butter and slowly whisk until just combined, add the egg yolks one at a time, whisking gently on low speed until just combined.
Whisk egg whites until white and fluffy, about 2 minutes, and using a spatula fold half of the egg whites into the batter, then the other half.
Butter a 9inch pie dish and pour the batter into it, smooth the surface using a spatula.
In a deep baking pan, pour 2 cups of hot water and place the pie dish on the pan. Bake for 15 minutes at 340F and without opening the oven reduce temperature to 320F and bake for another 15 minutes. Turn the oven off (do not open the oven) and let it rest in the oven for another 15 minutes.
Remove from oven, transfer dish to a wire rack and let cool completely to room temperature.