Preheat oven to 325 degrees, and grease a 12-cup Bundt pan with nonstick cooking spray.
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Sift flour, salt, and baking powder into a medium bowl and set aside.
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In a stand mixer fitted with the paddle attachment, cream together butter, vanilla and sugar on medium speed until very light and fluffy, about 3 to 4 minutes.
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Add the canola oil and beat until light and fluffy.
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Next, add eggs and egg yolks one at a time, and beat until fully incorporated, about 1 minute.
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Add the flour mixture and buttermilk to the batter in 3 alternating additions, starting and ending with the buttermilk. Beat just to combine.
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Spoon the batter into the prepared pan using a spatula to smooth the top.
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Bake for 60 minutes, or until toothpick inserted into cake comes out clean.
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Let cool in pan on wire rack for 10 minutes, then invert onto rack, remove the pan and let cool completely.
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Once cooled, turn the cake upside down on a cutting board.
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Using a melon baller, scoop out several mounds of cake through the underside, being sure not to cut through top or sides of cake.
For the filling:
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In a medium glass bowl, using a whisk, combine egg whites, sugar and cream of tartar.
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Place the bowl over a double boiler, and whisk constantly until sugar dissolves and the egg whites are just above room temperature, about 3 minutes.
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Remove bowl from the double boiler, and use an electric hand mixer fitted with a whisk attachment on medium-high speed to whip the mixture until stiff glossy peaks form, about 6 minutes.
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Add vanilla and whisk to combine.
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Using a large piping bag fitted with a large, round tip, fill the hollowed out sections of the cake.
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Flip the cake onto a serving platter and dust with powdered sugar before serving.