Talk about a throwback!

1 and 3/4 cups all-purpose flour
3/4 cup unprocessed unsweetened cocoa powder (plus more for dusting)
2 tablespoons baking soda
1 teaspoon baking powder
3/4 teaspoon Kosher salt
1 and 1/2 cups granulated sugar
1/2 cup brown sugar
1 cup sour cream
1/3 cup water
2 teaspoons vanilla extract
1/2 cup vegetable oil
3 large eggs
cooking spray
3/4 cup milk (chilled)
2 tablespoons cornstarch
1/2 cup unsalted butter (softened)
1/2 cup granulated sugar
1 teaspoon vanilla extract
1/8 teaspoon Kosher salt
1 (12-ounce) bag semi-sweet chocolate chips
1 cup unsalted butter (softened)
1 (8-ounce) package cream cheese (softened)
2 large egg yolks
1 teaspoon vanilla extract
3 cups powdered sugar (sifted)
1/8 teaspoon Kosher salt

For the Cake Bars: Preheat the oven to 350ºF. Spray an 9 x 13-inch baking pan with cooking spray and line the bottom with parchment paper. Spray the parchment paper with cooking spray and dust with cocoa powder and set aside.
In a large bowl, add sour cream, water, vanilla extract, vegetable oil, and eggs and whisk together until fully combined. Set aside.
In the bowl of a stand mixer fitted with the paddle attachment, sift together the flour, cocoa powder, baking soda, baking powder, and salt. Add granulated sugar and brown sugar and mix mixture on low speed until fully combined.
Increase speed to medium-low, and slowly add in sour cream mixture and mix until fully combined.
Pour mixture into prepared pan and place in the oven. Bake for 40-45 minutes, or until a toothpick inserted into the center comes out clean. Remove from the oven and allow to cool slightly.
On a clean workspace, invert pan to remove cake. Remove the parchment paper and discard. Cut cake into 12 equal size bars.
Transfer bars to refrigerator until ready to fill.
For the Filling: In a small saucepan, add milk and cornstarch and whisk together until fully combined. Place pan over medium heat and bring to a gentle simmer, stirring occasionally. Allow mixture to simmer until it has thickened, about 5-6 minutes.
Remove from heat, set aside and allow to cool completely to room temperature.
Meanwhile, in the bowl of a stand mixer fitted with the paddle attachment, add butter and sugar. Mix on medium speed until light and fluffy, about 2-3 minutes. Add vanilla and salt and mix until incorporated. Increase speed to medium-high and slowly pour in chilled milk mixture. Mix until mixture is light and fluffy, about 3 minutes. Transfer filling to a piping bag fitted with a straight pastry tip.
For the Frosting: In a medium microwave safe bowl, add chocolate chips and place in microwave for 30 seconds. Stir and return to microwave for another 30 seconds, or until chocolate is melted and glossy. Set aside and allow to cool.
In the bowl of a mixer fitted with the whisk attachment, add butter and cream cheese. Mix on medium speed until light and fluffy, about 3-4 minutes, stopping and scraping down the sides occasionally.
Add egg yolks and vanilla and mix until fully combined. Add powdered sugar, cooled chocolate, and salt. Mix until light and fluffy, about 3-4 minutes. Cover with plastic wrap and set aside until ready to use. When ready to use, transfer to a piping bag fitted with a 789 cake tip or straight edge tip.
To assemble: Line a baking sheet with parchment paper and set aside.
Using a serrated knife, cut a slit lengthwise into the top of each cake, stopping a 1/4-inch short of the edges. Pipe the prepared filling into the slit until the filling slightly bulges out and gently press the cake sides together. Transfer to prepared baking sheet. Repeat until all of the cake bars are filled.
Pipe chocolate frosting along the full length of the top of the filled cakes to cover completely. Serve immediately or transfer to refrigerator until ready to serve, up to 3 days.
Tip: If not using a 789 cake tip, run the tines of a fork along the icing to create ridges!