Prepare the pastry:
Preheat your oven to 425°F (220°C) and line a baking sheet with parchment paper.
In a bowl, sift together the flour, salt, and baking soda. Set aside.
In a heavy saucepan, heat the water and butter until the butter melts. Remove the pan from heat and add the flour mixture all at once. Stir quickly with a wooden spoon until well combined.
Return the pan to medium-high heat, stirring constantly until the dough pulls away from the sides of the pan and forms a smooth ball.
Remove from heat and add the eggs one at a time, beating well with a wooden spoon or mixer after each addition until the dough is smooth.
Fill a pastry bag fitted with a ½-inch round tip and pipe 3-inch rounds onto the prepared baking sheet, leaving about ½ inch of space between each puff.
Bake for 20 minutes, or until golden brown. When done, carefully slit the side of each puff to release steam, preventing them from becoming soggy inside. Transfer to a cooling rack and let cool completely.
Make the custard:
In a medium saucepan over medium heat, combine the sugar, flour, and salt.
Gradually whisk in the milk, cooking and stirring constantly until the mixture thickens and bubbles.
Reduce the heat and cook for 2 more minutes, stirring. Remove from heat.
In a small bowl, slowly whisk some of the hot mixture into the egg yolks to temper them. Then, return the egg mixture to the saucepan, cooking for another 2 minutes while stirring.
Remove from heat and stir in the butter, vanilla extract, and rum.
Transfer the custard to a shallow bowl, cover with plastic wrap (pressing it directly onto the surface to prevent a skin from forming), and refrigerate until fully cooled.
Assemble the cream puffs:
Once the custard is cooled, cut the tops off the cream puffs or use a piping bag to fill the slits you made earlier. Fill the puffs generously with custard until they are full.
Optional: Top with a cherry and dust with powdered sugar for a classic touch.