Set a medium heat-proof bowl over a small saucepan of simmering water to create a double-boiler. Add butter to bowl and heat until butter is mostly melted. Remove bowl from heat, and continue to whisk until butter is smooth and just melted.
Slowly stream milk into butter while whisking. Continue whisking until mixture is smooth, 2 to 3 minutes. If separation occurs, set bowl back on the double boiler and whisk until mixture is more homogenized.
In a medium microwave-safe bowl, melt butter in 20-second increments until mostly melted. Whisk butter until mixture is smooth, then slowly stream in milk while whisking. Continue whisking until mixture is smooth, 2 to 3 minutes.
If separation occurs, return bowl to microwave and heat for 5 to 10 seconds. Whisk until mixture is smooth and more homogenized. Repeat heating in 5-second increments if necessary.
Let heavy cream mixture cool for 5 minutes. Add in powdered sugar and resume whisking until mixture begins to thicken, about 4 minutes.
If mixture seems too loose, chill in the refrigerator for 5 minutes, then continue whipping until medium-stiff peaks form. Use immediately.
