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1
In a heavy- bottomed sauce pan with at least a 2-quart capacity, dissolve the sugar in the milk over medium-high heat.
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2
Once the sugar is dissolved, stir in the baking soda.
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3
Bring the milk to a near-boil, taking much care not to let it boil over. Just as it's coming to a boil, lower the heat to low and allow the mixture to simmer uncovered for 1-2 hours, stirring often (a silicone spoon or spatula is especially handy for this job). It should look like it's simmering a bit vigorously but not actually be at a full boil.
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4
Remove the pan from the heat when the milk becomes a dark golden color and appears quite thick. A good test to know when it's done is to stir the mixture with a spoon and run your finger along the back of the spoon (careful-- it's hot!); the trail your finger leaves should last several seconds before slowly disappearing.
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5
Allow dulce de leche to cool to room temperature.
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6
In a small bowl, whisk together the flours and baking powder.
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7
In separate bowl, cream butter, sugar, and salt.
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8
Add egg, milk, and vanilla to the sugar mixture; mix well.
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9
Add dry ingredients to wet ingredients, and mix until incorporated.
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10
Shape dough into a few smaller portions, flatten into discs, wrap in plastic, and refrigerate until stiff, at least one hour or overnight.
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11
Heat oven to 350u00b0F degrees. Line cookie sheets with parchment or silicone mats.
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12
Roll out dough to 1/4-inch thickness on a lightly floured surface. Cut into shapes and move to baking sheets (cookies won't spread much so they can be set quite close together).
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13
Re-roll and re-cut dough scraps giving dough a bit more time in the fridge as necessary.
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14
Bake cookies until lightly golden, about 10 minutes. Transfer to wire racks to cool.
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15
To assemble the cookies, spread a 1/4-1/2 teaspoon of cooled dulce de leche on the flat side of the cookie and use another cookie, flat side in, to complete the sandwich.
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16
Store any unused dulce de leche in an airtight container in the refrigerator.
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17
Store cookies in an airtight containter at room temperature.
