Place 1 cup granulated sugar and 1/3 cup dark brown sugar into a medium, heavy-bottomed saucepan and place over medium heat. In a separate, smaller saucepan, combine 3/4 cup heavy cream and 1/4 cup maple syrup and place over medium-low heat.
As the sugar heats, you will see it start to melt around the edges and turn golden in color, around 2 to 3 minutes. Using a heatproof rubber spatula, nudge the sugar slightly on one side. You will see liquid caramel slowly move out from underneath the granulated sugar. Very slowly start to move the sugar into the liquid caramel. Don't rush this! If the caramel starts to look grainy, you're incorporating the sugar too quickly. If the caramel starts to smoke or get very dark, remove the pan from heat until the smoking subsides and replace on lower heat.
Meanwhile, keep your eye on the cream and maple syrup mixture. You want it to come to a boil just when the sugar reaches the liquid caramel stage, so adjust the timing accordingly by raising or lowering the heat underneath the mixture. If the mixture comes to a boil too early, don't be afraid to remove it from the heat.
When the sugar has completely melted and is dark amber in color, remove from the heat and pour in the cream — it will bubble substantially. When the caramel has settled down, stir the mixture until smooth. Add 4 tablespoons cubed unsalted butter and 1 teaspoon kosher salt and stir until smooth, about 30 seconds.
Whisk in 1 teaspoon whiskey and zest from one medium-large orange. Transfer the caramel into a heatproof jar or container and let cool completely — as it cools, it will thicken considerably. The caramel sauce can be stored, covered, in the refrigerator for up to 4 weeks.