For the cake:
1 1/4 cups Dr Pepper (not diet)
1 cup rolled oats (not instant)
1/2 cup unsalted butter, room temperature
1/2 cup granulated sugar
1 cup brown sugar
2 large eggs, beaten
1/2 teaspoon vanilla extract
1 1/2 cups all-purpose flour
1 teaspoon baking soda
1/2 teaspoon ground cinnamon
Pinch of ground allspice
Pinch of ground nutmeg
1/4 teaspoon kosher salt

For the icing:
1/2 cup butter, melted
1/2 cup brown sugar
1/4 cup buttermilk or half and half
1/4 teaspoon ground cinnamon
Pinch of salt
1 cup shredded coconut
1 cup roughly chopped pecans