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1
Position a rack in the middle of the oven.
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2
Preheat the oven to 325 F. Line two baking sheets with parchment paper and butter the paper.
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3
In a large bowl, using an electric mixer on low speed, mix the flour, brown sugar, and baking soda to blend them.
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4
Add the butter and continue mixing until the butter pieces are the size of peas, about 2 minutes; you will still see some loose flour.
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5
Stop the mixer and scrape the sides of the bowl as needed during mixing.
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6
Mix in the egg and vanilla.
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7
The batter will still look dry.
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8
Mix in the sour cream and milk until the batter looks evenly moistened.
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9
You may still see lumps of butter.
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10
Mix in the chocolate chips.
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11
Drop heaping tablespoons (about 3 level tablespoons each) of dough at least 2 1/2 inches apart onto the prepared baking sheets, Bake one sheet at a time until the tops feel soft but firm, the edges are lightly browned and crisp, and a toothpick inserted in the center comes out clean, about 17 minutes.
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12
(If the toothpick penetrates a chocolate chip, test another spot.)
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13
Cool the cookies on the baking sheets for 10 minutes, then use a wide metal spatula to transfer them to a wire rack to cool completely.
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14
THE FILLING:
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15
In a medium saucepan, heat the cream and butter over low heat until the cream is hot and the butter melts.
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16
The hot cream mixture should form tiny bubbles and measure about 175 F on a thermometer; do not let the mixture boil.
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17
Remove the pan from the heat, add the chocolate chips, and let them sit in the hot cream mixture for about 30 seconds to soften.
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18
Whisk the filling until it is smooth and all of the chocolate is melted.
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19
Pour the chocolate filling into a medium bowl and let it sit at room temperature until thickened enough to cling to the cookies, about 45 minutes.
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20
Or, refrigerate it until it thickens slightly, about 20 minutes.
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21
Turn half of the cookies bottom side up.
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22
Leaving a 1/8-inch plain edge, Spread the filling over the bottoms of the cookies.
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23
If the filling is not thick and firm, let the cookies sit until the it firms up before adding the top cookie.
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24
Gently place the remaining cookies bottom side down on top of the chocolate filling.
