In a medium saucepan or pot, add the cold cubed butter, water, milk, salt, sugar, and vanilla. Heat over low heat until the butter melts and mixture simmers. Be careful not to boil at this stage, or your mixture may curdle.
1/2 (113g) cup unsalted butter, 1/4 (56g) cup water, 1/4 (56g) cup whole milk, 1 teaspoon vanilla extract, 1 (12g) tablespoon granulated sugar, 1/4 teaspoon fine sea salt