Preheat your oven to 350°F (175°C) and grease an 8x4-inch loaf pan, lining it with parchment paper for easy removal later.

In a medium saucepan over low heat, melt the butter, honey, and sugar together, stirring constantly until completely smooth and liquid. Remove the pan from the heat and let the mixture cool for about 5 minutes until warm but not hot.

In a large mixing bowl, whisk together the all-purpose flour, gingerbread spice mix, and baking soda. In a separate bowl, whisk the eggs and sour cream together until smooth and uniform.

Pour the cooled honey mixture and the egg mixture into the dry ingredients, stirring gently with a spatula just until combined and no flour streaks remain.

Pour the batter into the prepared loaf pan and bake for 45 to 50 minutes, or until a toothpick inserted into the center comes out clean. Let the cake cool in the pan for 10 minutes, then transfer to a wire rack to cool completely.

Slice the cooled cake horizontally into two even layers, spread the plum jam across the bottom layer, and place the top layer back on. Melt the dark chocolate and heavy cream together in a bowl until glossy, pour over the cake, and let it set for 15 minutes before serving.