Make the custard before or during baking. Follow my recipe for Ube Custard here. In a large saucepan, combine butter, water, milk, salt, and sugar. Stir until butter is melted and sugar is dissolved. Bring this to a simmer.
Lower the heat and add the flour all at once. Stir constantly until the mixture starts to clump up and form a ball. Then smash the ball against the sides of your pan in a thin layer to cook the mixture a little bit more for about 1 minute. Remove from heat.
Let the mixture cool for a few minutes. You can do this step manually or with a stand mixer. Add the eggs one at a time incorporating each egg fully before adding another, at least 30 seconds. Once all the eggs are added, you should end up with a nice choux pastry dough.
Preheat your oven to 400℉. Line your baking sheet with parchment paper. I like to draw circles on the underside of the parchment paper at even intervals to help with my spacing.
Fill a piping bag with your dough and pipe it onto your baking sheet in small, peaked mounds. Once all the piping is done, push down on the peaks to keep a flatter/rounded top after baking. Brush with egg wash
Bake in the 400℉ oven for 20 minutes. Then lower the temperature to 350℉ and back for another 10-15 minutes. If you haven't made your Ube Custard filling yet, do so now. Remove from oven and let cool just enough to handle. Poke each pastry with a knife to allow moisture to escape. Then let cool fully.
Slice your pastries in half and pipe the filling. Serve and enjoy!