Place the salted butter in a small skillet over medium heat.
Add the pecans, and cook, stirring, until toasted and fragrant.
Set aside to cool.
In a large mixing bowl, cream the unsalted butter and sugar together on medium-high speed, until pale and fluffy (about 5 minutes).
Beat in the eggs and vanilla until smooth.
Add the flour, baking powder, and salt, stirring on medium-low speed until incorporated.
Scrape the bottom and sides of the bowl with a silicone spatula, and stir in the milk until smooth.
Fold in the cooled buttered pecans.
Divide the batter equally between all 24 lined wells of the cupcake pans.
Bake for 18 to 22 minutes, or until a toothpick inserted in the thickest part of a cupcake comes out clean or with a few moist crumbs.
Cool completely, then top with brown butter Swiss meringue buttercream and garnish with chopped toasted pecans.
To make the brown butter Swiss meringue buttercream:
Place the butter in a small skillet over medium heat.
Cook until the butter melts, foams, and turns a nutty brown color.
Remove from heat and cool until solidified.
Place the egg whites and sugar in a large mixing bowl, and set it over a pot of barely simmering water.
Cook, whisking occasionally, until the mixture becomes thin and opaque, and the sugar has completely dissolved. (It should no longer feel gritty when rubbed between the thumb and forefinger.)
Remove the bowl from the heat and whisk on medium high speed until completely cool. (There should be no hint of warmth when you place your hand on the side of the mixing bowl. Refrigerate if necessary.)
When the meringue is completely cool, fluffy, white, and thick enough to hold stiff peaks, add the solidified brown butter, one tablespoon at time, while whipping on medium-high speed. (If it begins to look curdled, continue whipping until it comes together before adding more butter.)
Mix in the salt. (If the frosting seems runny, place it in the refrigerator for 20 minutes, then re-whip.)