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1
Position a rack in the middle of the oven.
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2
Preheat the oven to 350 F. Line two baking sheets with parchment paper.
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3
Put the chocolate in a heatproof container or the top of a double boiler, and place it over, but not touching, a saucepan of barely simmering water (or the bottom of the double boiler).
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4
Stir the chocolate until it is melted and smooth.
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5
Remove from the water and set aside.
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6
Sift the flour, baking powder, and salt into a medium bowl and set aside.
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7
In a large bowl, using an electric mixer on medium speed, beat the butter, brown sugar, and vanilla until smoothly blended, about 1 minute.
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8
Stop the mixer and scrape the sides of the bowl as needed during mixing.
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9
Add the eggs and mix until blended, about 1 minute.
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10
On low speed, add the flour mixture, mixing just until it is incorporated.
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11
Drop heaping tablespoons (about 3 level tablespoons each) of dough 3 inches apart onto the prepared baking sheets.
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12
Drizzle about 1/2 teaspoon of the melted chocolate over the top of each cookie.
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13
Using a small sharp knife, gently swirl the chocolate once or twice through the cookie to marbleize it.
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14
Bake the cookies one sheet at a time until the tops feel soft but set and the edges are lightly browned, about 11 minutes.
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15
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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16
The cookies can be stored in a tightly covered container at room temperature for up to 4 days.
