In a medium pot, whisk together water, egg yolk, and cornstarch. Add in brown sugar and chopped pecans and whisk to combine.
Cook over medium heat, whisking constantly, until mixture comes to a boil. Boil, whisking constantly for approx. 1 min.
Remove from heat and whisk in butter and vanilla. Cool completely before using on cake.
Brown Sugar Cake:
Preheat oven to 350F. Grease and flour two 8" cake rounds and line with parchment.
In a medium bowl, whisk flour, baking powder, and salt until well combined. Set aside.
Using a stand mixer fitted with a paddle attachment, cream butter and sugars on med-high until pale and fluffy (approx 3mins). Reduce speed and add eggs one at a time fully incorporating after each addition. Add vanilla.
Alternate adding flour mixture and milk, beginning and ending with flour (3 additions of flour and 2 of milk). Fully incorporating after each addition.
Bake for 35-40mins or until a toothpick inserted into the center comes out mostly clean.
Place cakes on wire rack to cool for 10mins then turn out onto wire rack and cool completely.
Brown Sugar Swiss Meringue Buttercream:
Place egg whites and sugar into the bowl of a stand mixer, whisk until combined.*
Place bowl over a double boiler on the stove and whisk constantly until the mixture is hot and no longer grainy to the touch (approx. 3mins). Or registers 160F on a candy thermometer.
Place bowl on your stand mixer and whisk on med-high until the meringue is stiff and cooled (the bowl is no longer warm to the touch (approx. 5-10mins)).
Switch to paddle attachment. Slowly add cubed butter and mix until smooth.**
Add vanilla and whip until smooth.
Place one layer of cake on a cake stand or serving plate. Pipe a damn of buttercream around the top and fill with approximately 2/3rds of the pecan pie filling.
Place second cake layer on top and apply a thin coat of frosting all over to crumb coat. Chill for 20mins.
Frost & smooth the top and sides of the cake. Do a rope border using a Wilton 6B piping tip. Chill for 20mins.
Fill the middle between the rope border with the remainder of the pecan pie filling. Decorate top and bottom with pecan halves if desired.
*Ensure there is NO trace of egg yolks in your whites and that your mixer bowl and whisk is completely grease free or your meringue won't stiffen. ** The buttercream may look like it's curdled at some point. Keep mixing until it is completely smooth.