Preheat oven to 350° F and line a muffin pan with cupcake liners.
In a small bowl, combine granulated sugar and lavender and rub with your fingers until fragrant and set aside.
In a medium bowl, whisk together flour, salt, and baking powder. Add a tablespoon of the flour mixture to the berries and toss to coat. In a glass measuring cup, combine milk and yogurt. Set all of this aside for now.
In the bowl of a stand mixer fitted with the paddle attachment, combine butter, brown sugar, and sugar and lavender mixture. Cream on medium speed until light and fluffy and no chunks of butter remain. Scrape down the sides of the bowl.
With the mixer on low speed, add eggs one at a time, followed by the vanilla extract and mix until smooth and homogenous.
With the mixer on low speed, alternate between adding the dry ingredients and the milk/yogurt mixture, starting and ending with dry ingredients (dry-wet-dry-wet-dry).
When the last of the dry ingredients have been added, remove the bowl from the mixer and scrape down the sides to ensure everything is well-combined. Use a rubber spatula to gently fold in the berries until they are evenly distributed.
Use a large cookie scoop to portion batter into your prepared muffin cups, sprinkle with demerara sugar and bake for 20-25 minutes until muffins are tall and golden brown, and a knife or toothpick comes out clean when inserted. Cool slightly and then enjoy warm, split open and slathered with lots of butter and flaky salt.