Soften butter for buttercream chips.

Beat softened butter until creamy.

Gradually add powdered sugar and salt to the butter.

Add milk, a teaspoon at a time, only if needed, until creamy.

Line a baking sheet with wax or parchment paper.

Spoon small dollops of buttercream onto the baking sheet.

Freeze the baking sheet with buttercream dollops for 20 minutes.

Chop the frozen buttercream into medium-sized chunks.

Return the chopped buttercream chips to the freezer.

In a large bowl, combine sweetened condensed milk, vanilla extract, and cake mix.

Stir until the cake batter is well blended.

Stir in the salt.

Whip heavy whipping cream until firm peaks develop.

Gently fold the whipped cream into the condensed milk mixture using a rubber spatula.

Fold in the buttercream chips.

Add rainbow sprinkles, if desired.

Transfer the ice cream mixture to a Tupperware container.

Freeze overnight.