Preheat the oven to 350°F (175°C). Lightly grease a 9 x 13-inch baking pan and line its base with parchment paper, leaving an overhang if you plan to lift out the cake.

Whisk flour, granulated sugar, cocoa powder, baking powder, baking soda, and salt in a large mixing bowl until evenly combined.

Add eggs, buttermilk, oil, and vanilla. Mix just until smooth. Slowly stir in hot coffee or hot water; the batter will look thin, which is normal.

Pour batter into the prepared pan. Bake for about 32–38 minutes, checking near 35 minutes, until the center springs back lightly and a toothpick comes out with a few moist crumbs but no wet batter.

Cool the cake completely in its pan on a wire rack for about 60 minutes. Do not frost it while warm.

Beat softened butter until creamy. Add powdered sugar in batches, then vanilla, salt, and 2 tablespoons milk. Beat until fluffy; add the remaining milk only as needed. Mix in optional cocoa powder for a light parchment tint.

Spread buttercream in a smooth, even layer over the cooled cake. Lightly trace a border and lettering guides with a clean toothpick, leaving room for YES, NO, the alphabet, 0–9, and GOOD BYE.

Pipe the board border, YES and NO, rows of alphabet letters, numbers, GOOD BYE, and optional sun-and-moon symbols with black decorating icing using a fine tip. Work slowly, cleaning the tip between details.

Roll the white fondant to about 1/4 inch thick on a lightly dusted surface. Cut or shape a small rounded triangular, heart-like planchette and remove a little circle near its pointed end with a clean straw. Place it on the cake.

Slice into 16 pieces with a clean knife, or cover and refrigerate until serving. For best texture, let refrigerated slices stand at room temperature for about 20 minutes before eating.