How to Make It
Preheat the oven to 325°. Butter three 9-inch round cake pans and line the bottoms with parchment paper. Butter the paper and dust with flour, tapping out the excess.
Sift the flour, cream of tartar, baking soda and salt into a medium bowl. In a small pitcher, combine the milk and vanilla. In a large bowl, beat the butter on medium speed until creamy. Slowly add the sugar and beat until light and fluffy, scraping down the sides of the bowl. On low speed, alternately add the dry ingredients and the milk in 3 batches. Beat the batter until smooth, scraping down the sides of the bowl as necessary.
In another bowl, using clean beaters, beat the egg whites until they form soft peaks. Stir one-third of the egg whites into the cake batter to lighten it. Using a rubber spatula, fold in the remaining egg whites until no white streaks remain.
Pour the batter into the cake pans and smooth the tops. Tap the pans lightly on a work surface to release any air bubbles. Bake the cakes on the middle and lower racks of the oven for about 30 minutes, shifting the pans halfway through, until the tops spring back when pressed lightly and a toothpick inserted in the center comes out with a few moist crumbs attached. Let cool in the pans for 5 minutes, then invert the cakes onto a wire rack to cool completely. Peel off the paper.
In a large saucepan, combine the egg yolks and sugar and stir until smooth. Add the melted butter and cook over moderate heat, stirring, until thick enough to coat the back of a spoon, about 6 minutes; do not let it boil. Stir in the pecans, raisins and coconut and cook for about 1 minute. Add the bourbon, vanilla and salt and let cool to lukewarm.
Place a cake layer on a serving plate, right side up, and spread with 1 1/4 cups of the filling. Repeat with a second cake layer and another 1 1/4 cups of filling. Top with the last cake layer and frost the cake with the remaining filling. Let the cake cool completely before serving.