To make the cake
Position a rack in the lower third of the oven and preheat oven to 325°F. Coat a 12-cup Bundt pan with nonstick cooking spray and dust it lightly with flour.
Sift together flour, baking powder, and salt into a large bowl.
In the bowl of a stand mixer fitted with the paddle attachment, beat butter and vanilla extract on medium speed until smooth and creamy. Add sugar and beat until evenly mixed, about 1 minute. Slowly pour in oil. Beat until light and fluffy, about 2 minutes. Add eggs and egg yolks, 1 at a time, beating thoroughly after each addition.
Reduce mixer speed to low. Stir in flour mixture and buttermilk in three alternating additions, ending with the buttermilk, and continue to mix on low speed until batter is smooth and no lumps remain. Turn off mixer and fold batter several times by hand to ensure everything is well incorporated, and then pour batter into prepared pan.
Bake for 60 to 70 minutes, until the cake is golden, the top springs back when lightly pressed, and a cake tester inserted into the center comes out clean. Place pan on a wire rack and let cool completely, about 2 hours.
To make the filling
For the filling, in the bowl of a stand mixer, beat together marshmallow crème and butter until smooth. Transfer to a pastry bag fitted with a large round tip.
With the cake still in the pan, use a paring knife or an apple corer to cut 6 or 7 deep holes into the bottom of the cake, each about 3⁄4 inch in diameter; be careful not to cut through top of cake. Discard (i.e., nibble) cake scraps. With your fingers, gently burrow a horizontal tunnel around the center of the cake, connecting the vertical holes.
Insert the tip of the pastry bag into each hole and squeeze in filling, tilting pastry bag back and forth as you work to encourage filling into the horizontal tunnel through the cake. When cake is filled, use a spatula to scrape away excess filling from the bottom of the cake. Quickly and carefully invert cake onto a serving platter. Dust with confectioners’ sugar, if desired, and serve.
