To make filling: drain pineapple over medium bowl for 10 minutes, reserving 1/4 cup of the juice. Place the pineapple, reserved juice, and 3 tablespoons sugar in a medium saucepan. Bring to a gentle boil over a medium-low heat; cook for 4-6 minutes, stirring occasionally, or until liquid is slightly thickened. Stir in 1/2 teaspoon of vanilla. Place in a small bowl; refirgerate to cool.
To make cake:Heat oven to 350. Grease and flour two 8x1 1/2 round cake pans. Line bottoms with parchement paper.
In a large bowl, beat butter and 3/4 cup sugar for 7-8 minutes or until light and fluffy. IN a small bowl, whisk together egg yolks and 1 1/2 teaspoons vanilla. Add to butter mixture in two parts, beating well after each addition.
In a medium bowl, stir together cake flour, baking powder, baking soda, and salt. Beat into butter mixture at low speed, alternating with buttermilk, beginning and ending with flour. Divide batter evenly into pans, smoothing with spatula.
Bake 15-20 minutes or until golden brown and center springs back when pressed. Cool on wire racks for 10 minutes, invert onto racks and cool completely.
Melt 3/4 cup butter in a medium saucepan over a medium heat. Add brown sugar; cook 2-3 minutes, stirring occasionally or until mixture comes together (brown sugar and butter will seem separated until mixture gets hot enough). Stir in milk and bring to a boil, stirring constantly. Pour into a medium bowl and cool for 30 minutes or until room temperature. Once mixture is cooled, sift in powdered sugar 1/2 cup at a time, whisking well after each addition. Stir in vanilla.
To assemble: Place cake layer on platter. If filling is juicy, brush excess juice over bottom cake layer. Spread layer of frosting over top of cake, top with filling. PLace top layer of cake on filling, pressing gently to allow th ecake and filling to come together. Frost top and sides of cake with remaining frosting.HOlding the cake in one hand, gently press chopped pecans into the sides of the cake using the other hand.