Preheat oven to 350°F. Grease and flour a 10-cup or 12-cup Bundt® pan (see note below for tips on preparing the pan). In a medium bowl sift together the sifted cake flour, baking powder, baking soda, and salt.
In the bowl of a stand mixer fitted with a flat beater, beat the butter and 1½ cups of the sugar until light and fluffy. Beat in the egg yolks one at a time. Beat in 2 teaspoons of the vanilla. On low speed, add the sifted dry ingredients alternately with the sour cream. Scrape down the sides of the bowl.
Transfer a third of the batter to a separate bowl and stir in ½ cup of the Sunday Night Chocolate Premium Dessert Sauce. (If the sauce is not at room temperature, transfer to a small microwave-safe bowl and heat in a microwave oven on high for 10 seconds.) Using a rubber spatula, stir in the chocolate sauce until fully incorporated.
Spoon a third of the plain batter into the prepared pan. Dollop half of the chocolate batter on top; spread evenly. Repeat with another third of the plain batter and remaining chocolate batter. Top with final third of the plain batter. With a table knife in a vertical position, gently swirl the batters together to create a marble pattern (do not overstir or the marble effect will lose its visual appeal). Smooth the top surface evenly.
Bake for 45 to 50 minutes or until a toothpick inserted near the center comes out with a few moist crumbs still sticking to it. Do not overbake. Transfer cake in the pan to a wire rack; let cool in the pan for 10 minutes. Loosen the cake by jiggling it side to side until it moves slightly. Place a piece of parchment paper over a wire rack. Invert cake onto the parchment paper (which will catch any drips from the glaze).
Meanwhile, to make the glaze, in a small saucepan combine water, the remaining ½ cup sugar, and the remaining ½ teaspoon vanilla. Bring to a boil, stirring constantly. Cook until sugar has dissolved. While the cake is still warm, poke holes in the cake with a toothpick. Use a pastry brush to cover the cake with glaze, applying a fourth of the glaze at a time and letting it soak into the cake between additions. Allow the cake to cool completely.
To serve, place the cake on a large platter. Transfer remaining chocolate sauce to a microwave-safe bowl and heat in a microwave oven on high for 10 seconds or until pourable. Pour sauce over cake. Cut into slices. Serve extra sauce on the side.
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