Before preparing the ice cream, heat-treat the Funfetti cake mix according to the package/manufacturer's food-safety guidance, then allow it to cool completely before using.
For easier whipping, chill your mixing bowl and beaters in the freezer for 10–15 minutes.
Add the chilled heavy whipping cream and vanilla extract to the chilled bowl. Beat with a hand mixer or stand mixer until soft peaks form.
In a separate large bowl, stir together the sweetened condensed milk, melted and cooled butter, and cooled, heat-treated Funfetti cake mix until well combined.
Gently fold the whipped cream into the condensed milk mixture until combined, being careful not to deflate the whipped cream.
Gently fold in the rainbow sprinkles.
Spread the mixture evenly into a loaf pan. Cover and freeze for at least 4 hours, or until firm.
Scoop and serve.
Notes
Chilling the mixing bowl and beaters for 10–15 minutes before whipping the cream can help the cream whip more quickly.Store the ice cream covered in the freezer for up to 1 month.