Step 1Make the filling: In a heavy saucepan, combine the milk and vanilla bean and bring to a boil over medium-high heat. Remove from heat, cover and set aside to steep for 15 to 20 minutes. Remove the vanilla bean, scraping out any seeds and adding to the milk (discard the bean or save it for another use).
Step 2In a heatproof medium bowl, whisk the egg yolks and sugar until blended. Sift in the potato starch and coffee, whisking until thoroughly combined.
Step 3In a heavy saucepan, bring the milk back to a boil. Remove from heat and gradually add the milk to the egg mixture, ladling and whisking it in to temper the eggs. Return the milk and egg mixture back to the saucepan and heat over medium-low heat, whisking constantly. Cook until the mixture begins to thicken, 2 to 4 minutes. Remove from heat and strain the mixture into a bowl set over a larger bowl of ice water, discarding any solids. Continue whisking until the custard is completely cooled and smooth. Transfer the custard to a covered container and refrigerate until completely chilled.
Step 4In the bowl of a stand mixer, beat the butter until soft and smooth, about 3 minutes. Slowly add the custard, beating in each addition until fully incorporated and smooth. This makes about 5 cups filling. Cover and refrigerate until firm, at least 1 to 2 hours.
Step 5Using a serrated knife, carefully cut off the top half of each puff and reserve this part as a "hat."
Step 6Fill the puffs: Using a pastry bag fitted with a large star tip, pipe the filling generously onto the bottom of each puff, allowing the cream to show slightly at the sides. Top each puff with its “hat.”
Step 7Sift powdered sugar over the cream puffs and serve.