Whisk egg yolks in a medium bowl until thick and lemon colored (about 1 minute).
Stir together sugar, flour, and salt in a large heavy saucepan; gradually whisk in half-and-half. Whisking constantly, bring mixture just to a boil over medium (about 8 to 10 minutes). Remove from heat.
Gradually whisk about one-fourth of hot sugar mixture into yolks; gradually add yolk mixture to remaining hot sugar mixture, whisking constantly. Return to heat; cook over medium, whisking constantly, 1 minute. Remove from heat, and whisk in butter, coconut extract, and 2 teaspoons vanilla extract. Transfer to a medium bowl, and place plastic wrap directly on warm filling; cool completely (about 2 hours).
Beat cream and remaining 1 teaspoon vanilla at medium-high speed with an electric mixer until foamy; gradually add powdered sugar, and beat until medium peaks form.
Reserve 12 vanilla wafers. Arrange half of remaining wafers in 1 layer in the bottom of a 3 1/2- to 4-quart bowl. Top with half of banana slices, half of custard, and half of whipped cream. Repeat layers once. Top with crumbled reserved wafers.