Preheat the oven to 350 degrees. Prepare the cake pans by lining them with parchment paper and spraying with non-stick spray. Dust with flour and tap out excess.
In a stand mixer, cream together the softened butter, brown sugar, and granulated sugar until smooth and fluffy.
Add eggs and vanilla to the butter-sugar mixture and mix well.
In a separate bowl, whisk together the flour, baking powder, and salt. Gradually add the dry ingredients to the wet ingredients, mixing thoroughly.
Pour in the milk to the flour mixture and mix until the batter is well combined.
Divide the cake batter equally between the prepared cake pans and bake for 35-40 minutes, until golden brown and a toothpick inserted into the center comes out clean. Let the cakes cool completely before assembling.
While the cakes bake and cool on wire racks – make the pecan pie filling and the frosting.
For the pecan filling – in a medium sauce pan, whisk together the water, egg yolk, cornstarch, dark brown sugar and pecans.
Cooking over medium heat and stirring occasionally, heat until the mixture comes to a boil. Whisking constantly, boil for one minute.
Remove from the heat and add in the butter and vanilla. Whisk until combined. Cool completely before filling the cake.
For the frosting – create a double broiler using the bowl of a stand mixer placed over a large pot filled with about 2 inches of water. Allow the water to come to a boil before starting the frosting.
In the bowl of the stand mixer placed over the pot in the double broiler, combine the brown sugar and egg whites. Whisking constantly, continue to heat the mixture for approximately 3-5 minutes. If you have a thermometer, the mixture should reach 160 degrees. Otherwise, dab the mixture and it should no longer be grainy. It will be very sticky!
Place the bowl in the stand mixer and whisk on medium speed for about 5-10 minutes or until stiff peaks is achieved and the bowl is no longer warm to the touch.
Switch the whisk attachment for a paddle attachment. While the mixer is going on low / medium speed, slowly add in cubes of butter, mixing well between each addition. The mixture may look curdled at some point – just keep mixing!!! Mix until smooth.
Add in the vanilla and whip until smooth.
To assemble – remove the cakes from their pans.
Place first layer on a cake board or serving plate. Pipe a circle of frosting around the outside top edge to create a dam. Using about half of the pecan pie filling, fill the center of the cake and spread evenly to the frosting around the edge. Top with the remaining second layer.
Using an offset spatula or similar, spread a thin layer of frosting over the top and sides of the cake. This is just a crumb coat to set the base for the final layer of frosting. Should just be thin enough to cover the cake and still see the cake underneath in thinner areas. Chill for approximately 15 minutes.
Remove the cake from the fridge and cover the cake in another layer of frosting. Smooth the top and then smooth the sides as best as possible. Place the remaining pecan pie filling in the center on top of the cake. Spread toward the edges and leave about a one inch perimeter around the sides.
Transfer the remaining frosting to a piping bag fitted with a large star tip. In one continuous flow, pipe a rope around the edge of the top layer – rotating the cake if needed.
Pipe a scalloped border or stars around the bottom of the cake to create a border.
With the remaining chopped pecan halves, sprinkle over the border around the bottom and to decorate the top.
