Final answer:
Marcel Desaulniers' cookbook 'Death By' highlights chocolate as the primary ingredient, featuring a rich variety of chocolate-based desserts and helping popularize the 'death by chocolate' concept.
Step-by-step explanation:
The sweet ingredient highlighted in Marcel Desaulniers' cookbook titled Death By is chocolate. The full title of the cookbook is Death by Chocolate: The Last Word on a Consuming Passion, which offers a clue to the primary ingredient featured throughout the book. Marcel Desaulniers is renowned for his expertise in creating decadent chocolate desserts, which is the main focus of this cookbook.
Recipes in Death by Chocolate take the reader on a journey through various indulgent chocolate concoctions, from simple cookies to elaborate cakes and everything in between. The cookbook is often credited with popularizing the 'death by chocolate' concept, wherein the recipes celebrate the rich flavor of chocolate in all its forms.