1. Make the Cream Puff Dough:

1. Preheat oven to 375°F (190°C) and line a baking sheet with parchment paper.
2. In a saucepan, combine water, butter, and salt. Bring to a boil over medium heat.
3. Reduce heat to low and add flour and baking soda, stirring vigorously until the dough pulls away from the sides of the pan (about 1-2 minutes).
4. Transfer the dough to a mixing bowl and let cool for 5 minutes.
5. Add eggs one at a time, beating well after each addition until the dough is smooth and glossy.
6. Transfer dough to a piping bag fitted with a large round tip and pipe 2-inch mounds onto the baking sheet.
7. Bake for 25-30 minutes, or until golden brown and puffed. Let cool completely before filling.

2. Prepare the Custard Filling:

1. In a saucepan, whisk together sugar, flour, and salt. Gradually add milk, whisking until smooth.
2. Heat over medium heat, stirring constantly, until the mixture thickens and begins to bubble.
3. In a separate bowl, whisk egg yolks, then gradually add a small amount of the hot milk mixture to temper the eggs.
4. Return the mixture to the saucepan and cook for 2 more minutes, stirring constantly.
5. Remove from heat and stir in butter, vanilla extract, and rum (if using). Let cool to room temperature, then refrigerate until chilled.

3. Assemble the Cream Puffs:

1. Cut each cooled cream puff in half.
2. Fill a piping bag with the chilled custard and pipe it into the bottom halves.
3. Place the top halves back on and dust with powdered sugar before serving.