Step One: Grate zest from lemons to equal 2 Tbsp. Cut lemons in half; squeeze juice into a measuring cup to equal 1 cup.
Step Two: Beat butter and sugar at medium speed with an electric mixer until blended. Add eggs, 1 at a time, beating just until blended after each addition. Gradually add lemon juice to butter mixture, beating at low speed just until blended after each addition; stir in zest. (Mixture will look curdled.) Transfer to a heavy 4-qt. saucepan.
Step Three: Cook over medium-low heat, whisking constantly, 14 to 16 minutes.
Step Four: Place heavy-duty plastic wrap directly on warm curd (to prevent a film from forming), and chill 4 hours or until firm. Store in an airtight container in refrigerator up to 2 weeks.