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1
Adjust oven racks to upper-middle and lower-middle positions and preheat oven to 350u00b0F. Line 2 baking sheets with parchment paper.
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2
In large bowl, whisk flour, sugar, cocoa, baking powder, baking soda, and salt; set aside. In medium bowl whisk together milk, oil, egg, butter, and vanilla. Pour wet ingredients into dry and whisk until smooth.
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3
Using a 1 1/2 ounce ice cream scoop, scoop out 18 to 20 mounds of batter on prepared sheets. Bake until firm to touch, 8 to 9 minutes, rotating pans from top to bottom and front to back halfway through baking. Transfer parchment sheets (with cakes still attached) to wire rack to cool completely.
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4
In small saucepan over medium heat, whisk milk with flour, cocoa powder, and salt until combined. Cook, gently whisking, until mixture thickens and just begins to boil (one to two large bubbles will pop to surface). Cover with plastic wrap and let mixture cool to room temperature.
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5
In bowl of standing mixer fitted with whip attachment, whip butter, shortening, sugar, and vanilla on medium-high speed until fluffy, about 2 minutes. Add cooked flour mixture and continue to whip until very light and fluffy, about 5 minutes. Reduce speed to low and mix in melted chocolate.
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6
Divide filling between half of the cakes. Sandwich together with remaining cakes. Serve.
