Gather in the culinary playground a stellar cast of ingredients - fresh lemon juice, a pinch of salt, cinnamon, cornstarch, dark rum, a dash of ground cinnamon and nutmeg, maple flavoring, unsalted butter, brown sugar, apple juice, combined dark and golden raisins, and a hint of allspice. Let the symphony of flavors commence!

Place the saucepan on the stage and let apple juice, raisins, sugar, spices, lemon juice, and salt twirl into a simmering waltz over medium heat. Allow this aromatic dance to unfold for a delightful 10 minutes, creating a fragrant medley that teases the senses.

Meanwhile, in a secret chamber, unite dark rum and cornstarch. Give it a vigorous whisk, conjuring a magical elixir that will soon transform our sauce into a decadent masterpiece.

Introduce the rum-cornstarch potion to the simmering sauce. Watch as it weaves its enchantment, thickening the concoction in a mere 2-3 minutes. A visual spectacle unfolds as the sauce transcends into a velvety, irresistible potion.

The grand finale approaches. Remove the saucepan from the heat, and with a flourish, add the unsalted butter. Stir until it gracefully surrenders, infusing the sauce with a luxurious, creamy texture that promises pure indulgence.

Behold your culinary opus! Serve this ambrosial raisin sauce over a warm upside-down cake or your dessert of choice. Each spoonful carries a melody of sweet, spicy, and buttery notes - a harmonious finale to your gastronomic symphony!