Line a 9” pan with parchment paper and spread the butter pecan ice cream into an even, compact layer. Cover with plastic wrap and place the ice cream back into the freezer to chill until firm– about 3 hours.
To make the cookies, add the softened butter and brown sugar to the bowl of a stand mixer fitted with the paddle attachment. Beat on medium speed until light and fluffy– about 2 minutes.
Next add the egg and vanilla. Mix to combine, scraping down the sides of the bowl as needed.
In a medium bowl, whisk together the flour, cocoa, baking powder, and salt.
With the mixer on low speed, slowly add the dry ingredients to the butter mixture. Mix just until the dry ingredients are fully incorporated– you don’t want to overmix!
Divide the dough into two discs and wrap in plastic wrap. Refrigerate the cookie dough for 30 minutes, or until firm but rollable.
Once the cookie dough has chilled, roll out each disc on a lightly floured surface until they are ⅛” thick. Cut out any shape you like– I'm making 1.5”x2.5” rectangles. If using the same size cookie cutter, you should yield 16 cookies. Carefully transfer the cookies to parchment-lined baking sheets, making sure to place them at least 2” apart.
Place the cookies in the fridge or freezer (either will work) to chill for 30 minutes. While the cookies chill, preheat the oven to 350°F/180°C.
Bake the chilled cookies for 12-14 minutes, or until they are just set around the edges. Remove the cookies to a wire rack and allow them to cool completely before continuing.
Make sure to work quickly when assembling the ice cream sandwiches– we don’t want them to melt! Using the parchment, lift the frozen ice cream out of the pan and place it on your work surface. Using the same cookie cutter from Step 7, cut out as many pieces of ice cream as you can. If you used the same size cookie cutter as I did, slice the ice cream block in half and then slice each half into 4 even rectangles and they should be just the right size.
Sandwich a slab of ice cream between two cookies and place the ice cream sandwich on a rimmed baking sheet. Repeat until all of the ice cream sandwiches are assembled.
Store the ice cream sandwiches covered in the freezer until ready to enjoy.