• Preheat oven to 350 degrees. Spray 2 (9-inch) cake pans with cooking spray with flour. Line bottom of pans with parchment paper and lightly spray parchment paper.
• Sprinkle the chopped pecans in the bottoms of the pans.
• Using an electric mixer, beat butter and dark brown sugar at medium speed until fluffy, about 3 to 4 minutes. Be sure to scrape down the sides of the bowl to evenly mix everything.
• Add eggs one at a time, beating well after each addition.
• In a medium bowl, whisk together flour, baking powder, and salt.
• Add flour mixture, alternating with milk, beating just until combined after each addition. (Add flour mixture in 3 additions and milk in 2).
• Beat in vanilla.
• Divide batter evenly between the prepared pans, smoothing out the top with a spatula.
• Bake 32 to 36 minutes or until a wooden pick inserted in the center comes out clean. Let cool in pans 10 minutes and then remove from pans to cool completely.

Make Filling

• In a medium saucepan, stir together pecans, sugar, corn syrup, melted butter, egg, egg yolk, vanilla, and salt.
• Bring to a boil over medium-high heat, stirring frequently. Simmer and stir for about 3 minutes. 
• Pour into a bowl and let cool to room temperature.
• Spread pecan filling on top of one cake layer. Place the second layer on top.

Make Brown Sugar Frosting

• In a small saucepan, bring brown sugar, water, and salt to a boil. Stir until sugar is dissolved. Add 1/2 cup butter, stirring until melted. Let cool to room temperature.
• Combine cooled brown sugar mixture and remaining butter in a mixing bowl and beat with an electric mixer until smooth.
• Gradually beat in powdered sugar.
• Beat in milk. Spread frosting on top and sides of cake.