Prepare Blueberry Sauce: In a small saucepan over medium heat, combine blueberries and sugar. Stir gently while cooking for about 5-7 minutes until the blueberries burst and thicken into a sauce. Let it cool.

Beat Butter and Milk: In a stand mixer or food processor, add softened butter and whole milk. Beat on low speed for about 1-2 minutes until combined and creamy. Scrape down the bowl midway.

Fluff the Mixture: Increase the mixer speed to medium-high and beat for 5 minutes until light and fluffy with a pale color.

Add Ingredients: With the mixer on low, add the cooled blueberry sauce, maple syrup, and kosher salt. Mix until just combined, avoiding overmixing.

Serve or Store: Serve immediately for best texture or store in an airtight container in the refrigerator.