Mix 2 1/2 pints ice cream in an electric mixer on medium-high speed until smooth. Quickly spoon ice cream into a 5 1/2 cup (5 1/2-by-5-inch) sand-castle mold, filling completely. Smooth surface. Cover with plastic wrap. Freeze until set, at least 10 hours (or up to 3 days).
Trim domed top from the chilled yellow butter cake to make it level. Cut cake in half horizontally. Place the bottom layer on a parchment-lined baking sheet. Mix remaining ice cream with the electric mixer until smooth.Working quickly, spread about 2 1/2 cups ice cream over bottom layer; top with second layer. Spread remaining ice cream over top and sides of assembled cake. Chill in freezer until set, at least 4 hours (or overnight).
Process cookies to form fine crumbs. Mound crumbs on a serving platter, reserving 1/4 cup for garnish. Reserve 16 pecan halves for garnish; break remaining nuts into small pieces.
Transfer assembled square cake to the platter, centering it on top of the crumbs. Dip castle mold into a bowl of lukewarm water for about 15 seconds, being careful not to let water reach the rim. Pat mold dry. Remove plastic wrap; unmold ice cream castle onto the center of cake.
Working quickly, line edges of castle with pecan halves and pieces. Chill cake in freezer at least 1 hour (or up to 5). Just before serving, sprinkle reserved cookie crumbs over castle. Let stand at room temperature 5 minutes before cutting.