Add yolks, 1/4 cup milk, sugar, vanilla extract, salt, corn starch in a bowl.
Bring the remaining 3/4 cup milk to a scald in a saucepan. Pour the hot milk in a small stream into the egg mixture, whisking constantly with a balloon whisk as you pour. Once incorporated, pour everything back into the saucepan.
Whisk the mixture over medium heat until it thickens and firm up. Remove the heat and whisk in butter.
Once it reaches room temperature, scoop the creme into a piping bag or container. Refrigerate until ready to use.
To make the cream puffs, use a small nozzle and attach it to the piping bag. Insert the nozzle into the choux pastry and pipe the creme into the puff.