Melt butter in a saucepan over medium-low heat. Add banana puree, brown sugar, lemon juice, vanilla extract, cinnamon, lemon zest, and salt. Cook, stirring constantly, until slightly thickened, 8 to 10 minutes.
Beat eggs and egg yolk in a bowl until foamy. Add 2 to 3 tablespoons hot banana mixture to temper the eggs. Slowly add egg-banana mixture to saucepan and cook, stirring constantly, until the back of a spoon is coated with the curd, 4 to 7 minutes.