Melt 2½ tablespoons butter with ½ teaspoon salt in a large skillet over medium heat. Add the chopped pecans and stir to coat. Toast, stirring frequently, until the butter browns and the pecans are fragrant. Transfer to a parchment-lined baking sheet and cool completely.

Whisk together the flour, baking powder, and kosher salt in a medium bowl. Set aside.

In the bowl of a stand mixer fitted with the paddle attachment, beat the softened butter and dark brown sugar on medium speed until a paste forms.

Add the egg and vanilla extract and beat well to combine.

Reduce speed to low and add the dry ingredients in three additions, mixing until almost fully incorporated before adding the next. Add the cooled buttered pecans with the final addition, reserving about 2 tablespoons for topping.

Scoop the dough into balls of approximately ¼ cup each. Press reserved pecans into the tops and sprinkle with fleur de sel. Refrigerate for at least 2 hours.

Preheat the oven to 325°F convection or 350°F conventional. Place dough balls 2 inches apart on parchment-lined baking sheets.

Bake 18 to 20 minutes, until the edges are golden brown and the tops look matte rather than shiny. Cool completely on the pan or a wire rack.