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1
Heat the oven to 350 degrees F and arrange a rack in the middle.
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2
Line a baking sheet with parchment paper; set aside.
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3
Combine flour, cocoa powder, baking soda, and salt in a medium bowl and whisk to aerate and break up any lumps; set aside.
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4
Combine butter and sugars in the bowl of a stand mixer fitted with a paddle attachment.
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5
Mix on medium speed until light in color and fluffy, about 3 to 4 minutes.
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6
Stop the mixer and, using a rubber spatula, scrape down the sides and bottom of the bowl.
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7
Add egg and vanilla, return the mixer to medium speed, and mix until smooth, about 1 minute.
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8
Reduce speed to low, add reserved flour mixture, and mix just until the dough comes together.
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9
Add toffee and pecan pieces and mix until they are evenly distributed.
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10
Scoop dough by rounded tablespoons, placing them 1 inch apart on the prepared baking sheet.
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11
Bake until cookies are set around the edges but still soft in the center, about 9 to 11 minutes.
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12
Remove from the oven and let cool to room temperature before removing cookies from the baking sheet, about 10 to 15 minutes.
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13
Repeat with remaining dough (you should have 24 cookies when youre done).
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14
Using a 3-tablespoon or 1-1/2-ounce ice cream scooper and working with 6 cookies at a time (3 bottoms and 3 tops), top each bottom cookie with 1 scoop of ice cream and 1 cookie.
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15
Firmly press the cookies together to evenly disperse the ice cream in the center (be careful not to break the cookies).
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16
Immediately wrap each sandwich with plastic wrap and place in the freezer.
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17
Repeat with remaining cookies and ice cream.
