In a large bowl, whisk together sugar and egg yolk until slightly fluffy for about 1-2 minutes.

Whisk in the cornstarch, flour, and vanilla until thoroughly incorporated, forming a smooth paste.

Heat milk in a saucepan on medium heat, then remove it as soon as it starts simmering.

Pour the warm milk over the egg yolk mixture slowly while whisking vigorously.

Pour the mixture back into the saucepan and cook on medium heat until it thickens, approximately 1 minute after the first boil.

If it gets lumpy, continue whisking until smooth and glossy. Strain if necessary.

Add softened butter in chunks and mix with a rubber spatula until incorporated.

Cool the pastry cream completely before using. Cover the surface with plastic wrap to avoid skin formation.

Once cooled, whisk the pastry cream to restore smoothness, being careful not to overmix.

Store any leftovers in an air-tight container in the refrigerator for 2-3 days.