1. Preheat oven to 350°. In a bowl, mix graham cracker crumbs, butter, and 1/2 cup sugar. Press over bottom and sides of 2 (9-in.) pie pans. Bake until pale golden, 5 minutes. If crusts shrink, gently push up with sides of a fork. Let cool.
2. Beat yolks, zest, and milk in a bowl with a mixer on high speed until thick, 5 minutes. Beat in whole eggs and lime juice just to blend. Divide mixture between crusts.
3. Bake pies until filling no longer jiggles when pies are gently shaken, 20 to 25 minutes. Cool on racks. Cover and chill at least 1 1/2 hours.
4. Beat crema and remaining 3 tbsp. sugar in a bowl with a mixer on high speed until thick, about 3 minutes. Spread over pies, swirling topping. Scatter more lime zest on top.
*Buy at a grocery store or Latino market.
Make ahead: Up to 1 day through step 3, covered. Add topping up to 4 hours before serving and chill, uncovered.
Note: Nutritional analysis is per serving.
Allow 3 1/2 hours to cool and chill.