1Heat the oven to 325°F and arrange a rack in the middle. Butter and flour the wells of the baking pan; set aside.
2In the bowl of a stand mixer fitted with a whisk attachment, beat egg yolks on medium speed, adding sugar a little at a time, until mixture is thick and light in color, about 5 minutes. In a separate large bowl, whisk egg whites until medium peaks form. Fold whites into yolk mixture until well incorporated. Sift flour and baking powder into batter, and fold just until incorporated. Mix in vanilla extract.
3Divide batter evenly among the baking pan wells and bake until cakes spring back when touched, a toothpick inserted into the center comes out clean, and cakes are golden brown, about 20 to 22 minutes.
4Remove from the oven and let cool for 5 minutes; then loosen from the sides by running a knife along the edge of the pan. Invert onto a rack, remove cakes, and cool completely.
5Once cakes are cool, flip each over, and with a straw or the end of a spoon make three incisions running the length of the cake. Scoop Vanilla Cream into a pastry bag fitted with a medium-size plain tip (or snip the corner off of a small plastic bag).
6Place the tip into each incision, and press cream into the incisions until full. When cakes are completely full, turn them back over and serve.