The day before you plan on making the cookies, brown the butter, Put it in a stainless steel skillet over medium-low heat and cook, stirring occasionally. Once melted cook a few minutes more; the butter will start to give off a nutty smell and turn a shade of goldish-brown. Remove from heat before it burns (you’ll know if it does), put in a clean container and refrigerate to re-solidify.
Beat until smooth and creamy. (I’d recommend a stand mixer, but since I don’t have one, I use an electric hand mixer.) Add the salt and sugar and beat until incorporated and fluffy. Beat in the vanilla and lavender.
Beat in the flour on a low-speed, mixing until completely incorporated.
Remover from the mixing bowl and flatten into a disc. Wrap with plastic and refrigerate at least an hour, or until firm.
Roll out the dough on a lightly floured surface to 1/4-inch thickness. Using a cutter of any shape (I like bunnies), cut cookies and place on a parchment-lined baking sheet. Sprinkle lightly with kosher salt and refrigerate once more, about 10 minutes.
Place in a 350 degree oven and bake until slightly browned at the edges; about 10 minutes. Remove from oven and re-locate to cooling trays.