Step 1

Arrange a rack in the center of the oven; heat to 350°F. Grease a 9x9-inch pan with cooking spray.

Step 2

In a large bowl, whisk the butter, granulated sugar, and brown sugar until well combined. Add the eggs one at a time, whisking to incorporate after each addition. Add the vanilla and stir to combine.

Step 3

Whisk in half of the buttermilk. Add the flour, oats, ground ginger, baking powder, cinnamon, salt, cardamom, cloves, and nutmeg and fold with a silicone spatula until the batter is smooth and evenly combined. Add the remaining ½ cup plus 2 tablespoons of the buttermilk and mix to combine. If desired, fold in the crystallized ginger (or reserve it to sprinkle over the batter).

Step 4

Pour the batter into the prepared pan and spread into an even layer (if you reserved the ginger, sprinkle it on top).

Step 5

In a medium bowl, mix the flour, brown sugar, oats, and salt to combine. Add the butter and use a pastry cutter or your hands to cut the butter into the flour mixture until moist crumbs form. Stir in the crystallized ginger. Sprinkle the streusel in an even layer over the surface of the cake.

Step 6

Bake the cake for 55 to 65 minutes, until a tester inserted into the center comes out clean or with a few moist crumbs attached.

Step 7

Let the cake cool in the pan before slicing and serving. Dust with powdered sugar, if using.