Grease the clean and dry coffin pans with a piece of solid fat such as Crisco or unsalted butter and place them in the refrigerator to allow the fat to set.

Place 2 egg yolks, 1 egg (including the white), and ⅛ teaspoon salt in a mixing bowl. Start by beating the eggs with an electric mixer. Begin at a low speed, then increase the speed. Gradually add both types of sugar (⅓ cup granulated sugar and ¾ cup powdered sugar).

Within 5-7 minutes of beating at high speed, you should have a thick, smooth, and light egg mixture in the bowl.

Remove the greased pans from the refrigerator. Using a spoon, fill each pan no more than halfway with batter. The batter will puff up in the oven. Place the filled pans on a baking sheet.

Preheat the oven to 300 °F. Put the coffin cookies on a sheet in the oven and bake for 10 minutes. As they bake, you'll notice the batter rising in the pans, creating a hollow space inside each cookie.

Next, reduce the oven temperature to a low 230 °F and bake the cookies for another 30 minutes. At this reduced temperature, the cookies will gently dry rather than bake. Within this half-hour, the batter will be fully baked, and you'll notice a subtle golden hue on the cookie surfaces.

Remove the baked cookies from the oven and let them cool for about five to ten minutes. Take each pan between your thumb and forefinger and gently tap the surface with the palm of your hand. The cookie should slide easily out of the pan.

Now comes the fun part - decorating the coffin surfaces with whipped cream! Start by spraying the whipped cream from one end of the cookie to the other, creating an elegant wavy pattern.