Prepare oven and baking dish:
Preheat oven to 350°F. Coat a 13- x 9-inch baking dish with cooking spray; set aside.
Prepare apples and crescent roll dough:
Cut each apple evenly into 8 slices. Unroll crescent roll dough, separating triangles and placing them side-by-side on a clean work surface.
Roll up dumplings:
Place 1 apple slice along the wide side of each triangle, and roll up until the apple slice is rolled into the dough. Repeat the process with the remaining apple slices and crescent roll dough. Filling baking dish:
Arrange wrapped apple slices in the prepared baking dish.
Make cinnamon-sugar sauce:
Melt butter, cinnamon, salt, and ¾ cup of the brown sugar in a medium saucepan over medium-low, whisking occasionally, until sugar is dissolved, 3 to 5 minutes.
Boil sauce:
Bring butter mixture to a boil over medium-low, whisking constantly, until mixture is slightly thickened, about 2 minutes.
Add brandy:
Remove from heat, and whisk in brandy.
Add sauce to baking dish:
Pour the butter mixture evenly over the crescent rolls in the prepared baking dish.
Add Mountain Dew:
Pour citrus-flavored soft drink around crescent rolls in prepared baking dish. Sprinkle the tops of the crescent rolls evenly with the remaining 2 tablespoons of brown sugar.
Bake dumplings:
Bake in preheated oven until crescent rolls are evenly browned and apples are tender, 35 to 40 minutes.
Cool slightly and serve:
Remove from oven and let stand in baking dish for 5 minutes before serving. Top warm dumplings with whipped cream and serve immediately.