Preheat the oven to 375F degrees. Prick the sweet potato a few times with a paring knife and place on a small baking sheet. Bake until the sweet potato is very tender, 60 to 90 minutes. Allow the potato to cool until it is warm enough to handle (or completely cooled) and then peel and place in a medium mixing bowl. To the potato, add melted butter and honey and mix with a whisk until well combined. Add eggs, brown sugar, and kefir and mix until combined.

Reduce the oven temperature to 350F degrees.

In a medium bowl, whisk to combine flour, baking powder, baking soda, salt, cinnamon, ginger, and five spice. Add the dry ingredients to the wet and mix with a flexible spatula until no more dry bits of flour remain.

To make the streusel, in a medium bowl mix to combine melted butter, brown sugar, flour, oats, and salt with a fork until hydrated and crumbly.

Line a 8-inch square baking pan (or a round pan) with parchment paper. Spread the cake batter in the pan into an even layer and top with the streusel topping. Bake until the cake is set, test by sticking a toothpick into the center and see if it comes out clean or with a few tiny crumbs attached, 50 to 55 minutes. Allow the coffee cake to cool for about 15 minutes in the pan and then transfer to a wire rack or platter to completely cool.

To make the glaze, mix to combine powdered sugar, melted butter, cream or milk, salt and vanilla until it is thick and smooth but still runs off the whisk. Adjust by adding a little more powdered sugar or cream to get the right consistency. Since there is butter in the glaze, it will firm up as it cools so make sure to drizzle while the glaze is still able to drizzle. If it does firm up just pop it into the microwave for a few seconds.

Drizzle the glaze over top of the coffee cake. Slice and serve or serve in an airtight container for up to 4 days.