Prepare the dried culinary lavender. To grind the dried lavender buds, put them in a zip-top bag and crush them with a rolling pin.
Cream the butter. Add the lavender and vanilla. In the bowl of a stand mixer fitted with the paddle attachment, beat the butter on medium speed. After it's a bit creamy (after ~1 minute), add 1 teaspoon of ground dried lavender and the vanilla. Mix until creamy, about 1 more minute.
Add the powdered sugar and milk. Add 3 ½ cups (400g) of powdered sugar and 2 Tablespoons of milk. Beat on low speed for 30 seconds, then increase to medium-high speed for 2 minutes. Scrape down the sides and bottom of the bowl.
Adjust as needed. If the frosting is too thin, add up to ½ cup more powdered sugar, ¼ cup at a time. If the frosting is too thick, add more milk, ½ Tablespoon at a time. If the lavender flavor is lacking, add 1 additional teaspoon of dried lavender. Add a pinch of salt if it tastes too sweet (⅛ teaspoon). In all scenarios, beat together, taste, and adjust as desired.
Color the frosting. Once your desired flavor and consistency is achieved, add in 1 part pink and 1 part violet gel food coloring using a toothpick. Start by dipping the end of the toothpick into the food coloring jar and apply a small amount directly in the buttercream. Mix until evenly colored. Always use a new, clean toothpick if adding more color!
Use immediately or store for later. If you're not going to use the frosting right away, it's best to store it in the refrigerator in an airtight container for up to 1 week.