Step 1

In a medium bowl, whisk together flour, oats, brown sugar, cinnamon, ginger, and salt.

Step 2

Add butter and work together using your fingers until butter is completely mixed in and no dry spots remain. Place the crumble in the refrigerator to chill while you make the rest of the cake.

Step 3

Preheat oven to 350°F and line a 9x13” baking dish with parchment paper.

Step 4

In the bowl of a stand mixer fitted with the paddle attachment, combine butter, granulated sugar, and brown sugar. Beat on medium speed until light and fluffy, about 2 minutes, making sure to scrape down the sides and ensure all butter chunks are mixed in.

Step 5

Meanwhile, in a medium bowl, whisk together flour, rye flour, baking powder, baking soda, salt, cinnamon, cardamom, nutmeg, and ginger. Set aside.

Step 6

With the mixer on low speed, add eggs one at a time, mixing each completely before adding the next. Scrape down the sides of the bowl and then add mashed sweet potato and vanilla extract. Mix on low-medium speed for 1-2 minutes more, until everything is combined and homogenous.

Step 7

With the mixer on low-speed, add 1/3 of the dry ingredients, followed by half of the buttermilk. When the buttermilk has mixed in, add another 1/3 of the dry ingredients, followed by the remaining half of buttermilk, then the remaining 1/3 of the dry ingredients. Scrape down the sides of the bowl and then mix one more minute. Remove bowl from mixer and use a rubber spatula to gently fold the batter a few more times, ensuring that the batter is smooth and homogenous and everything is well combined.

Step 8

Pour batter into prepared baking dish and use an offset spatula to smooth into an even layer. Remove crumble from refrigerator and sprinkle evenly over the top of the cake.

Step 9

Bake for 45-50 minutes, rotating the pan halfway through the baking time, until a cake tester or toothpick comes out clean when inserted into the center of the cake. Let cool completely in the pan while you make the glaze.

Step 10

Place butter in a small saucepan set over medium heat and cook for 6-7 minutes, swirling occasionally, until butter is foamy and fragrant and small amber flecks appear on the bottom of the pan.

Step 11

Quickly remove the butter from the heat and pour into a heatproof bowl. Immediately whisk in powdered sugar, salt, and heavy cream, adding more heavy cream if needed to achieve your desired consistency. Drizzle glaze over coffee cake and let set before slicing.