Filling: Preheat oven to 325 degrees. In the bowl of a food processor, combine cream cheese and sugar; pulse to combine. Add egg, sour cream, and flour; process until smooth. Pulse in chocolate. (You should have 1 1/2 cups.)

Cake: Butter a 14-cup Bundt pan. Dust with flour, tapping out excess. Whisk together flour, baking powder, and salt. In a separate bowl, whisk eggs lightly, then whisk in milk.

In the bowl of a stand mixer fitted with the paddle attachment, beat sugar and vanilla seeds on low speed to combine. Add flour mixture; beat to combine, about 30 seconds. Continue beating while gradually adding butter until crumbly, about 3 minutes. Slowly add half of milk mixture; increase speed to medium-high and beat until fluffy, about 1 minute. Slowly add remaining milk mixture, scraping down sides of bowl as needed. Beat until incorporated, about 30 seconds more.

Dollop half of batter into pan, smoothing top with an offset spatula. Create a 1-inch-deep well around center with the back of a spoon. Spoon filling into ring (it will overflow -- that's okay), then dollop with remaining batter, spreading with spatula to cover. Bake until a tester inserted in center comes out clean, about 1 hour. Let cool in pan on wire rack 15 minutes. Turn out onto rack; let cool completely, about 1 1/2 hours. Place cake on rack on a rimmed baking sheet lined with parchment.

Ganache Glaze: Place chocolate in a heatproof bowl. Heat cream and vanilla in a small pot until simmering; pour over chocolate and let stand 5 minutes. Whisk until smooth and chocolate has melted. Pour glaze over cake, allowing it to drip down sides. Refrigerate until set, about 30 minutes. Cake is best served the same day at room temperature, but can also be refrigerated, wrapped in plastic, up to 2 days.