Fast well, feast well. Sunday was traditionally the feast day (even in Lent).
You do fast before Mass, traditionally. Now I think they only require no eating one hour or more before Mass, unless medically necessary to eat.
Went from no food after midnight until Mass, to 4 hours before, now to one hour before.
Nowhere anywhere does it say, afaik, that you cannot enjoy dessert or take the time to make a dessert on a Sunday.
One should not be working excessively on Sunday, and one should maintain prayerful attitude.
If you were cooking desserts as a job, this would be an issue. If you were cooking desserts as a job in a hospital, it’s likely a bit more necessary to have cooks working Sundays for hospitals, so whatever the boss asks of you, you’d do. Ie patients need to be able to eat.
So maybe it depends on the context, but I think as you mean it, there is no issue whatsoever. Cook before Church or after, but maybe enjoy the dessert only after Mass is done, unless you go to Mass late in the day.