Pour half-and-half and cream into medium saucepan and heat just until scalded (tiny bubbles will form around edge of pan).
Mix liqueur, ginger, and salt in small bowl. Lightly whisk eggs together in separate mixing bowl. Add liqueur mixture and whisk to blend. Very gradually stir in scalded cream. Stir in sugar, pumpkin, and lemon zest.
Pour custard into six 3/4-cup custard cups or ramekins and place in baking pan. Pour enough hot water into pan to come halfway up sides of cups. Cover with cookie sheet. Bake until knife inserted in center comes out clean, 40-45 minutes.
Transfer custard cups to wire rack to cool completely. Cover with plastic wrap and refrigerate at least 2 hours. Serve cold.