On your work surface, make a well with the flour. Sprinkle half a teaspoon of salt around the edge. In the center, add the sugar, eggs, and the yeast mixture. Mix everything together. Once the dough forms, add the butter in small cubes and knead until fully combined. Knead for 1 or 2 minutes, then cover and let rise until doubled in size, about 30-40 minutes.

Once risen, shape the dough into small balls (they will grow a lot during baking). Arrange them close together on a greased baking sheet so they stay soft and connected after baking. Let the dough balls rest, then gently flatten them with your palm and use your fingers to make a small well in the center. You can also use a tool to shape them if you have one.

Let them rest for another 5 minutes. Prepare the egg, milk, sugar, and salt mixture for brushing. Brush the dough balls, but do not brush the center where the pastry cream will go. Pipe the pastry cream into the center using a piping bag if you have one. Bake at 400°F (200°C) for 30 minutes, or until golden on top and bottom and still moist in the center. Sprinkle with sugar before baking.

Pastry Cream: In a small pot, whisk together the eggs and sugar. Add the milk, reserving half a cup to mix with the flour and cornstarch. Mix the flour and cornstarch with the reserved milk, then add to the pot. Cook over low heat, stirring, until it starts to bubble. Remove from heat, add vanilla extract, mix well, and cover with plastic wrap or a bag touching the surface of the cream.