Preheat the oven to 350 F, and position a rack in the center of the oven. Line two standard sized muffin pans with 18 paper cupcake liners.

In the bowl of your stand mixer fitted with the paddle attachment, beat the butter, sugar and lemon zest for about 5 minutes on medium speed, scraping the bowl down occasionally, until light and fluffy.

Add the egg whites, and beat for 1 minute, until well combined.

In a bowl, sift together the flour, salt and baking powder, then whisk in the lavender until everything is evenly distributed.

In a separate bowl or glass measuring cup, whisk together the milk, sour cream and vanilla extract.

With the mixer on low, add the flour mixture in three additions, alternating with the milk mixture, starting and ending with the flour. Increase the speed to medium, and mix for 15 seconds.

Gently fold in the chopped blackberries with a spatula.

Divide the batter between the paper liners. Bake the cupcakes for about 18-22 minutes, until the tops spring back when gently touched, or a toothpick inserted comes out clean.

Cool the cupcakes in the pan for several minutes, then gently transfer to a cooling rack to cool completely before frosting.

In the bowl of your stand mixer fitted with the paddle attachment, beat the butter on medium speed for 1 minute, until smooth. Add the blackberry jam and the vanilla, and beat for 1 minute until smooth.

With the mixer on low, gradually add the powdered sugar by spoonfuls, the meringue powder, salt and lavender.

Increase the speed to medium, and whip for 4-5 minutes, scraping the bowl several times, until very light and fluffy. Turn the speed down to "stir" and mix for 1 more minute.

Fit a piping bag with tip 1M, and fill half full of buttercream. Pipe the buttercream onto the cooled cupcakes in swirls, rosettes and hydrangea flowers. See this post on How to Decorate Cupcakes 6 Ways with Tip 1M.

Decorate with blackberries, dried lavender, nonpareils and sparkling sugar.