Instructions

For the bourbon–brown sugar cake: Adjust oven rack to middle position and heat oven to 325 degrees. Grease two 9-inch round cake pans, line with parchment paper, grease parchment, and flour pans. Whisk flour, baking powder, and salt together in bowl. Heat milk, butter, and bourbon in small saucepan until butter is melted. Off heat, add vanilla. Cover to keep warm.
Using stand mixer fitted with whisk attachment, whip sugar and eggs on high speed until light and airy, about 5 minutes. Remove bowl from mixer. While whisking continuously by hand, slowly add milk mixture until incorporated. Whisk in flour mixture until incorporated.
Divide batter evenly between prepared pans and smooth tops with rubber spatula. Bake until tops of cakes are light brown and toothpick inserted in centers comes out clean, about 25 minutes, rotating pans halfway through baking. Let cakes cool completely in pans on wire rack, about 2 hours. Run thin knife around edges of cakes. Invert cakes onto wire rack and discard parchment.
For the candied pecans: Line baking sheet with parchment. Bring all ingredients to boil in medium saucepan over medium heat. Cook, stirring constantly, until water evaporates and sugar appears dry, opaque, and somewhat crystallized and evenly coats pecans, about 5 minutes. Reduce heat to low and continue to stir until sugar turns amber color, about 2 minutes longer. Transfer pecans to prepared sheet and spread into even layer. Let cool completely, about 10 minutes. Chop pecans coarse.
For the bourbon syrup: Bring sugar, bourbon, and water to boil in small saucepan over medium heat; cook until sugar dissolves, about 2 minutes.
For the bourbon–brown sugar frosting: Whisk brown sugar, cream, and bourbon in bowl until combined. Microwave, whisking every 15 seconds, until smooth, about 1 minute. Set aside.
Using stand mixer fitted with whisk attachment, whip butter and salt on medium speed until smooth, about 1 minute. Reduce speed to low and slowly add confectioners' sugar. Increase speed to medium and whip until smooth, about 2 minutes, scraping down bowl as needed. Add marshmallow crème, increase speed to medium-high, and whip until light and fluffy, 3 to 5 minutes. Reduce speed to low and slowly add brown sugar mixture. Increase speed to medium-high and whip to incorporate, about 1 minute, scraping down bowl as needed. (Frosting can be refrigerated for up to 24 hours; let come to room temperature before using.)
Using skewer, poke 30 holes in top of each cake layer. Brush bourbon syrup generously over top and sides of each cake layer (use all of syrup). Set aside 1 cup frosting for piping rosettes. Combine 1½ cups remaining frosting with ½ cup chopped candied pecans in bowl. Place 1 cake layer on platter. Spread pecan-frosting mixture evenly over top, right to edge of cake. Top with second cake layer, press lightly to adhere, then spread remaining 2½ cups frosting evenly over top and sides of cake. Refrigerate until frosting is firm, about 30 minutes.
Fill pastry bag fitted with star tip with reserved 1 cup frosting. Pipe rosettes (spiraling from inside out) around edge of cake. Sprinkle remaining candied pecans in even layer inside ring of rosettes, covering all exposed frosting. Serve.