Preheat the oven to 350°F. Spray a 9x13-inch baking dish with baking spray, set aside.

Place 4 ounces semi-sweet chocolate chips in a medium-sized bowl. Warm ½ cup whole milk in the microwave until it reaches 180-190°F, watch it carefully. Pour hot milk over the chocolate chips and let it sit for 5 minutes.

Whisk until combined.

Add ¾ cup sour cream and whisk until smooth, set aside.

In a medium-sized bowl, stir together 1 ½ cups cake flour, ½ cup unsweetened cocoa powder, ¾ teaspoon baking soda and ½ teaspoon fine sea salt. Set aside.

In a separate large bowl, with an electric hand mixer, mix 1 ½ cup light brown sugar, packed with ½ cup vegetable oil until combined.

Add 2 teaspoons vanilla extract and 3 large eggs (one at a time) until incorporated.

Add half of the dry mixture, mix to combine.

Add half of the chocolate/sour cream mixture, mix to combine. Repeat with the remaining dry and wet ingredients.

Pour cake batter into the prepared baking dish. Bake for 30-35 minutes until a toothpick inserted into the center comes out clean. Let cool completely.

While the cake is cooling, make the filling.

Add 1 cup whole milk and 5 Tablespoons all-purpose flour to a small saucepan, whisk to combine. Place over medium heat and cook, stirring constantly until thickened to a paste. Let this mixture cool to room temperature.

Add ½ cup unsalted butter, softened to room temperature and ½ cup vegetable shortening to a large bowl, whip with an electric hand mixer until combined.

Add 4 cups powdered sugar a little at a time until fully mixed in, the mixture will be thick.

Add the cooled milk/flour paste and vanilla to the bowl. Add in 1 teaspoon vanilla extract. Stir it in until incorporated and then whip it for 3 minutes on medium-high speed until it resembles a whipped cream-like consistency.

Smooth this over the cooled cake. Place the cake into the fridge while you assemble the ganache topping.

Place 12 ounce bag semi-sweet chocolate chips into a medium-sized bowl. Warm 1 ½ cups heavy cream in the microwave until it reaches 180-190°F, watch it carefully. Pour the hot cream over the chocolate chips. Wait 5 minutes and whisk until smooth then stir in 1 teaspoon vanilla extract. Let this mixture stand for 10 minutes to cool slightly.

Pour the ganache over the top of the cake, there may be a little bit leftover depending on how deep your pan is.

Place the cake back into the fridge, uncovered, for 45 minutes to set. Slice and serve.