Preheat the Oven

Twenty minutes or longer before baking, set oven racks in the upper and lower thirds of the oven and preheat the oven to 350°F/175°C.

Toast and Prepare the Pecans

Spread the pecans evenly on a baking sheet, set the pan on the upper rack, and bake for about 5 minutes, or just until light brown, to enhance their flavor. Stir once or twice to ensure even toasting and avoid overbrowning. Cool completely.

Divide the pecans into two equal parts. Leave one part whole and chop the other part into medium-fine pieces.

Make the Meringue

In the bowl of a stand mixer fitted with the whisk beater, beat the egg whites and brown sugar on medium speed until well mixed, about 1 minute. Scrape down the sides of the bowl. Raise the speed to medium-high and beat for about 5 minutes, or until very thick and light in color. Add the whole and chopped pecans and, using a silicone spatula, fold them into the meringue.

Shape the Cookies

Use a tablespoon to drop 10 dollops of the pecan meringue (0.6 ounces/17 grams each) onto each of two prepared cookie sheets, at least 1½ inches apart. Using a small offset spatula, shape the dollops into ½ inch high by 2¼ inch wide discs. They will spread to about 2½ inches but will not widen on baking. Stir the mixture from time to time to ensure that each spoonful includes some of the pecans. If necessary, use a metal spatula to coax the meringue into a more even disc.

Bake the Cookies

Bake for 10 to 15 minutes. The meringues will crack slightly in an attractive manner. Check for doneness. A metal cake tester inserted into one of the cracks will come out sticky and the cookies will give slightly to pressure. (An instant-read thermometer should read about 190°F/88°C.) If more baking is required, rotate the cookie sheets halfway around, reverse their positions from top to bottom, and continue baking for a few more minutes.

While the first batch of meringues is baking, shape the second batch of meringues.

Cool the Cookies

Set the cookie sheets on wire racks and let the meringues cool completely on the sheets. They will firm on sitting and become easy to lift off the parchment using a small pancake turner. Sandwich them with ice cream or store the unfilled meringues airtight for up to 2 months.

Sandwich the Cookies with Ice Cream

Assemble only half of the ice cream sandwiches at a time to keep the ice cream from melting. Refrigerate the ice cream for 20 minutes before sandwiching the cookies to soften it slightly. Set sheet pans or cookie sheets in the freezer to chill.

Set 10 of the cookies, bottom sides up, on one of the chilled sheet pans. Set scoops of ice cream, about ¼ cup in size, on top of each cookie. Place a second cookie, bottom side down, on top. When the ice cream is soft enough to press down easily, evenly press the top of each cookie until the ice cream comes almost to the edge. The ice cream should be about ½ inch thick.

Cover the sandwiches with a sheet of plastic wrap and set them in the freezer until firm. Then wrap each sandwich separately in plastic wrap and store in an airtight container in the freezer. Repeat with the remaining cookies.