In this churro inspired dessert, sour cream cinnamon bundt cake is swirled with chocolate for a moist and tender marble cake. Wonderful as is, but even better when topped with a brown butter cinnamon icing!
Preheat oven to 350F. Pull out your 10-12 cup bundt pan.
In a standing mixer fitted with a flat beater, cream together butter, granulated sugar, and brown sugar on medium speed until light and fluffy, 3-4 minutes. Scrape down the sides of the bowl with a silicone spatula and add the vegetable oil. Mix on medium until well combined.
Add the eggs and yolks one at a time, mixing for about 30 seconds in between each. Scrape down the bowl as needed. Add vanilla and mix in.
Turn the mixer to low so that you can begin adding the flour and sour cream. Add 1/3 of the flour and mix until just incorporated. Then 1/2 of the sour cream, 1/3 of the flour, 1/2 of the sour cream, and finish with the remaining 1/3 of flour. Each time mix until the addition is mostly mixed in. When you are done, take the bowl off of the mixer and finish mixing in the flour with your spatula in a few swift strokes.
Fill a medium saucepan with 1-2 inches of water. Heat the water until it's steaming but not quite simmering. Fill a heat proof bowl which fits snugly over the saucepan with the bittersweet chocolate chips. Melt the chocolate, stirring occasionally with a silicone spatula, being careful not to let the water boil.
Scoop out about 2 cups of the cake batter into a bowl. Mix the batter with the melted chocolate.
Spray your bundt pan with a flour and oil baking spray, or grease the pan with oil and then dust it with flour, tapping out the excess.
Dollop spoonfuls of both the yellow batter and chocolate batter into the the bundt pan so that you have layers of yellow and chocolate filling the pan. Then take a long skewer and give the batter a stir, doing a few gentle curly-cues around the pan.
Bake the cake for 60-75 minutes, or until a long skewer comes out clean with only a few crumbs attached.
Rest the cake for about 20 minutes in the pan. Then gently loosen the cake around the edges and carefully invert the cake onto a cooling rack. Let cake cool completely.
Mix together confectioners' sugar, cinnamon, and vanilla in a medium bowl.
Heat butter in a saucepan over medium heat. Once the butter has melted and has stopped bubbling up and splattering but has begun to foam, watch it carefully. Once the flecks of milk solids on the bottom turn brown and the butter smells nutty, take it off the heat. Immediately pour the butter into the bowl with the sugar.
Whisk to combine, adding 1-2 tablespoons of milk to the icing. IMPORTANT - USE ICING WHILE IT'S HOT. As soon as your icing comes together, drizzle it over the cake.