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1
For the cupcakes: Preheat the oven to 350 degrees F. Line two 12-cup muffin tins with paper liners and spray with cooking spray.
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2
Whisk the flour, cocoa powder, baking powder, baking soda and salt in a medium bowl; set aside.
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3
Whisk the buttermilk, food coloring and vanilla in a spouted measuring cup; set aside.
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4
Beat the granulated sugar and butter in a stand mixer (or a large bowl if using a hand mixer) on medium speed until very light in color and fluffy, about 5 minutes, scraping down the sides of the bowl as needed.
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5
With the mixer still on medium, slowly add the eggs and beat until fully incorporated.
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6
Reduce the speed to the lowest setting; with the mixer running, add 1/3 of the flour mixture, then 1/2 of the buttermilk mixture, then 1/2 of the remaining flour mixture, then the remaining buttermilk mixture, then the remaining flour mixture.
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7
Scrape down the sides and beat until well mixed.
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8
Fill the prepared liners about 3/4 full of batter.
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9
Bake until the cupcakes bounce back when pressed and a toothpick inserted in the center comes out clean, 20 to 22 minutes.
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10
Let cool in the pan for a few minutes, then transfer to a rack to cool completely.
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11
For the frosting: Put the flour in a small saucepan.
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12
Vigorously whisk in about 1/2 cup milk, making sure to get the whisk into the edges of the pan, until you have a smooth, thick paste.
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13
(This step keeps the flour from clumping.)
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14
Slowly whisk in the remaining milk until fully incorporated and the mixture is smooth.
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15
Cook over medium heat, whisking constantly, until the mixture is very thick, about 5 minutes.
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16
(Toward the end of the process, the mixture will become a thick paste; it may seem to be forming lumps, but whisk vigorously and the lumps will disappear.)
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17
Scrape into a bowl, press plastic wrap onto the surface and refrigerate until cool, about 45 minutes.
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18
Using a stand mixer fitted with the paddle attachment (or a hand mixer), beat the butter and granulated sugar on medium speed until very light and fluffy, about 5 minutes.
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19
Add the cooled flour mixture a tablespoon at a time and beat until smooth.
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20
Switch to the whisk attachment (or continue with the hand mixer), add the vanilla, salt and 3 to 5 drops green food coloring and whip until very light and fluffy, like thick whipped cream, up to 10 minutes with a stand mixer or 12 minutes with a hand mixer.
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21
To assemble: Using a sharp serrated knife, cut 3 cupcakes in half vertically, gently sawing through the paper liners; set aside.
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22
Frost the remaining cupcakes.
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23
Put the sanding sugar on a small plate and roll the edge of each cupcake 3/4 of the way around in the sugar, making a green border around each with a gap on one side.
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24
On a large serving platter, arrange 18 cupcakes in groups of three, pushing them together at the gaps, to make 6 shamrock cakes.
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25
Dip the tops of the cupcake halves in sugar and wedge a pointy end of 1 half between 2 of the cupcakes in each group to form the stems.
