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1
Position a rack in the centre of the oven and preheat the oven to 350 degrees F/180 degrees Celsius Grease two large baking sheets.
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2
In a medium bowl, gently whisk together the flour, baking powder, baking soda and salt.
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3
Set aside.
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4
In an electric mixer, using the paddle attachment, beat the butter at medium speed until creamy, about 30 seconds.
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5
Gradually beat in the sugar, mixing well until combined.
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6
Add the egg and vanilla extract and mix until blended.
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7
Scrape down the sides of the bowl.
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8
Alternately add the sour cream in two additions and the flour mixture in three additions, blending well after each.
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9
Using a 1/4 cup ice cream scoop or measure cup, scoop mounds of the dough onto the baking sheets, spacing them 3 inches apart.
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10
Moisten your palm to prevent sticking and pat the mounds into 2 1/2-inch disks.
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11
Bake the cookies, one sheet at a time, for 15 to 17 minutes, until just beginning to turn light golden brown.
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12
Let the cookies cool on the baking sheet for 2 minutes, then transfer them to a wire rack to cool completely.
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13
Make the White Glaze:
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14
In a small bowl, whisk together the confectioners' sugar, heavy cream, and vanilla extract until smooth.
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15
Cover the bowl and set aside.
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16
Make the Chocolate Glaze:
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17
Place the chocolate, heavy cream, and corn syrup in the top of a double boiler over barely simmering water.
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18
Heat, stirring occasionally, until the chocolate is melted and the glaze is smooth.
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19
Let the glaze cool until tepid, about 10 minutes.
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20
Ice the Cookies:
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21
Using a small offset metal spatula, spread the white glaze on half of each cookie.
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22
Let the glaze set for 10 minutes.
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23
Using the spatula, spread the chocolate glaze on the other half of each cookie.
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24
Let the cookies set at room temperature for at least 45 minutes.
