In a medium saucepan, combine the heavy cream, milk, 1/2 cup sugar, and a pinch of salt. Warm over medium heat, stirring occasionally, until the sugar dissolves and the mixture is hot but not boiling.
In a separate bowl, whisk together the egg yolks and the remaining 1/4 cup sugar until well blended.
Gradually temper the egg yolks by adding a small amount of the hot cream mixture, whisking constantly. Repeat until about half of the cream mixture has been added.
Pour the egg yolk mixture back into the saucepan with the remaining cream mixture. Cook over low heat, stirring constantly, until the mixture thickens enough to coat the back of a spoon (about 170-175°F).
Remove from heat and strain through a fine-mesh sieve into a clean bowl. Stir in the vanilla extract, mashed bananas, and lemon juice. Cover and chill in the refrigerator for at least 4 hours, or overnight.
Once chilled, churn the mixture in an ice cream maker according to the manufacturer’s instructions.
During the last few minutes of churning, add the crushed Nilla Wafers to the ice cream.
Transfer the ice cream to a freezer-safe container, layering it with additional crushed Nilla Wafers. Freeze until firm, at least 4 hours or overnight.
Serve garnished with more Nilla Wafers and optional whipped cream.