To make eclair filling:

1. Heat milk in microwave for 2 minutes until hot but not boiling.
2. In a medium saucepan, whisk together milk, sugar, cornstarch and salt over medium low heat. Whisk continuously until smooth and thick (this will take about 5 minutes.)
3. Add egg yolk and whisk for 1 more minute. Remove from heat and cool slightly, stirring every so often.
4. In a small bowl, mash banana and peanut butter together with potato masher until well incorporated. Whisk into cooled filling. Put in refrigerator until ready to use.

To make choux:

1. Preheat oven to 375 degrees F.
2. In a medium saucepan, add water and butter, bring to a boil.
3. Reduce heat to medium low, add flour and mix briskly until incorporated. Keep mixing until the mixture becomes a dough ball in center of pan. Remove from heat and cool for 5 minutes.
4. One at a time, mix eggs into dough, mixing very briskly.
5. To assemble eclairs, use ungreased cookie sheets and pastry bag. Pipe into rectangles 4-inches long, 1-inch wide and 3/4-inch high.
6. Bake for 25 -30 minutes until golden. Cool on a rack and then split length wise.

To assemble eclairs (before making glaze):

1. Fill bottom half of eclair with about 3 tablespoons of filling. Place top of eclair on top of filling like a sandwich.

To make glaze:

1. In a small saucepan, melt chocolate and peanut butter over medium low heat, stirring often. Remove from heat and cool for a few minutes.
2. Spoon about 2 tablespoons of glaze over top of eclair.