01 - Beat softened butter and cream cheese in a large mixing bowl with an electric mixer until smooth and creamy, about 2 minutes.
02 - Incorporate vanilla extract, ground cinnamon, ground nutmeg if using, and a pinch of fine sea salt; mix to blend evenly.
03 - Gradually add sifted powdered sugar while mixing on low speed until fully combined.
04 - Increase mixer speed to medium and beat mixture until light and fluffy, approximately 2 to 3 minutes.
05 - Add milk or heavy cream one tablespoon at a time until frosting reaches desired spreading consistency.
06 - Apply immediately to cooled cakes or cupcakes, or refrigerate covered for up to 3 days; re-whip before use if chilled.