In a medium saucepan, whisk together ¼ cup water, 1 large egg yolk, and 1 tablespoon cornstarch until smooth. Stir in 1 cup light brown sugar and 1 cup chopped pecans, cooking until it reaches a gentle boil. Continue whisking for 1 minute until thickened, then remove and mix in 2 tablespoons unsalted butter and 1 teaspoon vanilla extract. Allow to cool completely.

Preheat oven to 350°F (175°C) and prepare two 8-inch pans. In a bowl, whisk together 2 cups flour, 1 tablespoon baking powder, and ½ teaspoon salt. In a mixer, cream together ½ cup unsalted butter, 1 cup light brown sugar, and ½ cup granulated sugar until fluffy. Beat in 3 large eggs one at a time, then alternate adding the flour mixture with 1 cup milk until combined.

Pour the batter into prepared pans. Bake for 35-40 minutes until a toothpick comes out clean. Cool in the pans for 10 minutes before transferring to a wire rack.

In a stand mixer, combine 4 large egg whites with 1 cup dark brown sugar. Set over a double boiler, whisking until hot and sugar dissolves. Remove from heat and whip until stiff peaks form. Gradually add 1½ cups unsalted butter, mixing until smooth, then stir in 1 teaspoon vanilla extract.

Place one layer of cake on a stand, pipe buttercream around the edge, and spoon ⅔ of the pecan pie filling inside. Place the second layer on top, apply a thin crumb coat, and chill for 20 minutes.

Frost the cake with remaining buttercream, create borders, fill with leftover pecan filling, and garnish with pecan halves.