Preheat oven to 375 °F (191 °C). Lightly grease a 2.5 quart casserole dish with cooking spray and set aside.
In a large pot, bring water to a boil, season with salt, and add the sweet potatoes and yukon gold potatoes. Cook until fork tender, about 10 minutes.
Carefully drain the potatoes and put through a potato ricer or mash together with a potato masher in a large bowl.
Melt ½ cup (114g) butter in a small stockpot on stove and add the sage. Cook until fragrant, about 1 minute, careful not to burn it. Pour milk and butter mixture over potatoes and stir to combine.
In a medium, microwave safe bowl, melt remaining 2 tbsp. (28g) butter and add the panko. Mix until all breadcrumbs have been moistened.
Pour potato mixture into casserole dish and sprinkle breadcrumbs on top.
Bake uncovered for 45 minutes, or until mixture is bubbly. If topping is browning too quickly, tent with foil.
Serve warm.
You may make this up to 2 days in advance and reheat in the oven.