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1
Make batter: Preheat oven to 350F Using a small brush, lightly coat three 2 1/2-inch 6-cup muffin pans with melted butter.
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2
Dust with flour, tap out excess, and set aside.
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3
Place 2 cups flour, 1 1/4 cups milk, sugar, brown sugar, cocoa, 3/4 cup butter, sour cream, eggs, 2 teaspoons vanilla, baking powder, baking soda, and salt in a large bowl.
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4
Beat using a mixer set on low speed until combined.
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5
Increase mixer speed to medium and continue to beat 3 more minutes.
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6
Bake cakes: Divide batter equally between prepared pans and bake on middle rack of oven until cake tops spring back when lightly touched, 20 to 25 minutes.
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7
Cool in muffin pans on a wire rack 5 minutes.
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8
Use a knife to loosen cakes from pan sides.
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9
Invert cakes onto wire rack and cool completely.
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10
Make cream filling: Whisk remaining milk and flour together in a medium saucepan until smooth.
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11
Place saucepan over medium heat and cook, stirring constantly, until thick, about 5 minutes.
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12
Remove from heat, cool to room temperature, and set aside.
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13
Using an electric mixer set on high speed, beat remaining butter and confectioners' sugar together in a medium bowl until light.
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14
Add remaining vanilla, reduce mixer speed to medium, and slowly add cooled milk mixture.
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15
Beat 1 more minute.
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16
Fill cakes: Fill a pastry bag fitted with a long, narrow tip (we used a Bismark tube #230) with cream filling.
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17
Insert tip into bottom of cake and fill by gently moving tip from side to side while squeezing pastry bag, until cake begins to swell.
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18
Gently remove tip and return cake to wire rack.
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19
Fill remaining cakes and set wire rack over a baking pan.
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20
Glaze cakes: In a double boiler set over simmering water, melt bittersweet chocolate and shortening together.
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21
Once melted, remove pan from heat and let sit 3 minutes.
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22
Pour about 1 tablespoon chocolate mixture on top of a cake.
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23
Use a small spatula to glaze top and smooth around sides until completely coated in a thin layer of chocolate.
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24
Repeat with remaining cakes.
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25
Freeze 3 minutes, remove, and let sit until chocolate has set, about 1 hour.
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26
Decorate cake tops with remaining cream filling, if desired.
