Prep the oven: Preheat the oven to 325°F.
Grease the bundt pan: Dip a paper towel into a generous amount of shortening and spread it around the pan, making sure to coat all its nooks and crannies. Lightly dust the greased pan with flour (using a sift works well for this). Tap and rotate the pan to create a thin, even coating of flour across the pan’s interior. Gently tap out any excess flour over the sink.
2-3 Tbsp. shortening, 2 Tbsp. flour
Mix dry ingredients: Combine the cake flour, cornstarch, baking powder, and salt together in a large bowl. Set aside.
3 cups cake flour, 2 Tbsp. cornstarch, 1 ½ tsp. baking powder, 1 tsp. kosher salt
Cream ingredients: In the bowl of a stand mixer fitted with a paddle attachment, cream the softened butter, oil, and sugar on medium speed for ~3-5 minutes or until butter becomes noticeably lighter in color and fluffy. Scrape down sides of bowl as needed.
1 cup unsalted butter, 1 ½ cups granulated sugar, 2 Tbsp. vegetable oil, 1/2 cup light brown sugar
Whisk sour cream: Meanwhile, whisk together the sour cream and milk in a measuring cup or small bowl.
1 cup sour cream, 3/4 cup whole milk
Add eggs & vanilla: Decrease mixer to low and beat in the eggs & egg yolk, one at a time. Beat each egg just long enough for the yellow to disappear (about 30 seconds). Then add vanilla extract.
3 eggs, 1 large egg yolk, 1 Tbsp. vanilla extract
Add flour: Gradually add the flour mixture alternately with the sour cream mixture, beginning and ending with the flour mixture. Beat just until combined after each addition. Overmixing can lead to dry, tough cakes.
Add chocolate chips: Place the mini chocolate chips in a small bowl and toss them with ~2 teaspoons of flour until evenly coated. This light coating helps prevent the chocolate chips from sinking to the bottom of the cake as it bakes. Gently fold the flour-coated chocolate chips into the batter until evenly dispersed.
1 cup mini chocolate chips, 2 tsp flour
Fill bundt pan: Transfer the batter to the prepared bundt pan and gently tap the pan on the counter a few times to release any air bubbles.
Bake: Bake until a skewer inserted into the cake comes out clean or with only a few moist crumbs, ~60-65 minutes. Start Timer Be careful not to over bake. Cake will continue to cook in pan after you pull it out of the oven.
Cool: Cool the cake in pan for ~10-20 minutes. Start Timer Cooling allows cake to slowly begin to release itself from the sides of the pan. Resist the urge to pull it out immediately.
Make the simple syrup while the cake cools: Pour sugar and water into a small saucepan. Bring to a boil and stir until all sugar has dissolved. (1-2 minutes). Remove from heat and stir in vanilla. Set aside.
1/4 cup granulated sugar, 1/4 cup filtered water, 1 tsp. vanilla extract
Invert cake: Place a parchment paper-lined rack or baking sheet over the pan, then flip both over in one motion. Slowly lift the pan from the cake. YAY!
Add simple syrup to cake: Poke holes all around the cake with a fork or toothpick and brush the syrup over the cake using a pastry brush, letting it soak into the holes. Allow the syrup to sink into the cake, then add more syrup. Repeat until all syrup is used.
Make vanilla bean frosting: Allow the cake to cool, then make the vanilla bean frosting. Whisk ingredients for glaze together into a medium-sized bowl, it should be thick but still pourable.
1 cup powdered sugar, 3 Tbsp. heavy cream, 1 tsp. vanilla bean paste, pinch of salt
Frost cake: Pour over the cake. Garnish with more mini chocolate chips if desired. Let sit until set before slicing the cake and serving. Enjoy!