01 -
Preheat oven to 350°F. Grease a 9x13-inch baking dish with baking spray and set aside.
02 -
Place chocolate chips in a medium bowl. Heat milk in microwave until 180–190°F, then pour over chocolate chips and let stand for 5 minutes. Whisk until smooth, then add sour cream and whisk until combined. Set aside.
03 -
In a separate bowl, sift together cake flour, cocoa powder, baking soda, and salt. Set aside.
04 -
Using an electric mixer, beat brown sugar and vegetable oil until combined. Add vanilla extract and eggs one at a time, mixing well after each addition.
05 -
Add half of the dry ingredients to the wet mixture and mix until combined. Add half of the chocolate-sour cream mixture and mix. Repeat with remaining dry and wet ingredients until fully incorporated.
06 -
Pour batter into prepared pan. Bake on middle rack for 30–35 minutes or until a toothpick inserted into the center comes out clean. Allow to cool completely.
07 -
Whisk milk and flour in a small saucepan. Cook over medium heat, stirring constantly until thickened into a paste. Remove from heat and cool to room temperature.
08 -
In a large bowl, beat butter and shortening until combined. Gradually add powdered sugar until fully mixed. Add cooled milk-flour paste and vanilla extract, then beat on medium-high speed for 3 minutes until light and fluffy.
09 -
Spread the filling evenly over the cooled cake. Refrigerate while preparing ganache.
10 -
Place chocolate chips in a bowl. Heat heavy cream until 180–190°F and pour over chocolate. Let sit 5 minutes, then whisk until smooth. Let cool for 10 minutes.
11 -
Pour ganache over the filled cake, spreading evenly. Refrigerate uncovered for 45 minutes to set before slicing and serving.