The day before (or a few hours in a pinch), mix lavender and sugar. Place in a sealed container so that the lavender can flavor the sugar.

Using a handheld or stand mixer with a whisk attachment, beat the egg and sugar/lavender mixture together on high for 8 minutes. The mixture will turn pale and thicken as it beats. After 8 minutes, beat in the vanilla until incorporated.

In a separate bowl, whisk the flour, baking powder, and salt together. Using a spatula or wooden spoon, gently fold half of the flour mixture into egg mixture. When that is mostly combined, add and fold in the remaining flour. Be gentle to avoid deflating the egg mixture.

Heat or microwave the 4 Tablespoons of butter until melted but not hot. Stir about 1/4 cup of the batter into the melted butter and mix until well combined (this will deflate). Pour the butter mixture into the remaining batter and stir carefully until combined, deflating as little as possible.

Cover the batter and chill in the refrigerator for 30-60 minutes. Longer is not better because the butter may solidify, and the madeleines will get less fluffy.

Melt the remaining Tablespoon butter. Using a pastry brush, lightly brush the pan with melted butter (even if you have a non-stick pan). This will help create a crisper, “browned butter” exterior.

Remove the batter from the refrigerator. Taking care not to deflate it, spoon a rounded Tablespoon of batter into the center of each scalloped well. There should be enough for all 12 wells and it will spread as it cooks.

Bake for 10-12 minutes at 350 F, turning the pan halfway through if your oven heats unevenly. The madeleines are done when the tops spring back after lightly pressed with your finger and the edges are turning golden. Invert the pan onto the counter. Transfer the warm madeleines to a wire rack to lightly cool (your madeleines will stick if placed on a plate to cool).

Sprinkle with powdered sugar, if desired. Eat the same day if possible.