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1
Combine the butter, sugar, and glucose in the bowl of a stand mixer fitted with the paddle attachment and cream together on medium-high for 2 to 3 minutes.
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2
Scrape down the sides of the bowl, add the eggs and vanilla, and beat for 7 to 8 minutes.
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3
(See page 27 for notes on this process.)
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4
Reduce the mixer speed to low and add the flour, milk powder, cream of tartar, baking soda, salt, and rainbow sprinkles.
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5
Mix just until the dough comes together, no longer than 1 minute.
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6
(Do not walk away from the machine during this step, or you will risk overmixing the dough.)
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7
Scrape down the sides of the bowl with a spatula.
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8
Still on low speed, add the birthday cake crumbs and mix in for 30 secondsjust until they are incorporated.
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9
Using a 2 3/4-ounce ice cream scoop (or a 1/3-cup measure), portion out the dough onto a parchment-lined sheet pan.
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10
Pat the tops of the cookie dough domes flat.
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11
Wrap the sheet pan tightly in plastic wrap and refrigerate for at least 1 hour, or up to 1 week.
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12
Do not bake your cookies from room temperaturethey will not bake properly.
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13
Heat the oven to 350F.
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14
Arrange the chilled dough a minimum of 4 inches apart on parchment- or Silpat-lined sheet pans.
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15
Bake for 18 minutes.
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16
The cookies will puff, crackle, and spread.
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17
After 18 minutes, they should be very lightly browned on the edges (golden brown on the bottom).
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18
The centers will show just the beginning signs of color.
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19
Leave the cookies in the oven for an additional minute or so if the colors dont match and the cookies still seem pale and doughy on the surface.
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20
Cool the cookies completely on the sheet pans before transferring to a plate or an airtight container for storage.
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21
At room temp, the cookies will keep fresh for 5 days; in the freezer, they will keep for 1 month.
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22
In a pinch, substitute 25 g (1 tablespoon) corn syrup for the glucose.