Spiced Oatmeal Crumb Cake

A Cake for When the Seasons Start to Change

There’s always a point toward the end of summer when my mind starts wandering toward fall. I'm not necessarily ready for full-on pumpkin and apple season, but suddenly I want oats, warm spices, brown sugar, and the kind of cake that practically demands a cup of coffee alongside it.

This oatmeal spice crumb cake hits exactly that note.

The rolled oats are soaked in boiling water before being folded into the batter, which gives the cake an incredibly tender, moist crumb. Cinnamon, ginger, and mace add warmth, while brown sugar gives it a deeper caramel flavor.

Then there’s the crumble—which might be my favorite part. Rolled oats, oat flour, raw sugar, butter, and fresh vanilla create a craggy topping with plenty of crunch against the soft cake underneath.

It's simple, cozy, and exactly the sort of cake I find myself wanting as summer starts slipping into fall.

Ingredients

Oatmeal Spice Cake

1 cup (90 g) old-fashioned rolled oats

¾ cup (180 g) boiling water

½ cup (113 g) unsalted butter, softened

¾ cup (150 g) granulated sugar

¾ cup (150 g) packed light brown sugar

2 large eggs (about 100 g out of the shell), room temperature

2 teaspoons (8 g) vanilla extract

1 cup (120 g) all-purpose flour

¼ cup (30 g) oat flour

1 teaspoon (6 g) baking soda

1½ teaspoons (4 g) ground cinnamon

1 teaspoon ground ginger

½ teaspoon ground mace

½ teaspoon (3 g) fine sea salt

Vanilla Oat Crumble

¾ cup (180 g) cold unsalted butter, cubed

½ cup (100 g) raw or turbinado sugar

⅓ cup plus 1 tablespoon (50 g) all-purpose flour

¼ cup (30 g) oat flour

1 cup (80 g) old-fashioned rolled oats

½ teaspoon (3 g) fine sea salt

½ vanilla bean, split and seeds scraped

If you don't have raw or turbinado sugar, substitute ¼ cup plus 2 tablespoons (75 g) granulated sugar and 2 tablespoons (25 g) brown sugar. I highly recommend the raw sugar if you have it—the larger crystals give the crumble a particularly good crunch.

Method

Preheat the oven to 350°F (175°C). Grease a 9-inch square baking pan and line with parchment paper if desired.

Soak the Oats

Place the rolled oats in a bowl and pour the boiling water over them. Stir to combine and let the oats sit for about 20 minutes, until softened and hydrated. Set aside to cool while you prepare the crumble and cake batter.

Make the Vanilla Oat Crumble

Combine the raw sugar, all-purpose flour, oat flour, rolled oats, and salt in a bowl.

Scrape the seeds from the vanilla bean directly into the dry ingredients. Use your fingertips to rub the vanilla into the sugar, breaking up any clumps and distributing those tiny seeds throughout the mixture.

Add the cold, cubed butter and work it into the dry ingredients until everything is thoroughly moistened and you have a mixture of small crumbs and larger, craggy pieces.

Refrigerate the crumble while you make the cake batter. Keeping that butter cold helps the crumble hold onto its texture in the oven.

Make the Cake

In the bowl of a stand mixer fitted with the paddle attachment, cream the softened butter, granulated sugar, and brown sugar until light and fluffy.

Add the eggs one at a time, mixing well after each addition. Add the vanilla, scrape down the sides and bottom of the bowl, and mix again until everything is well incorporated.

In a separate bowl, whisk together the all-purpose flour, oat flour, baking soda, cinnamon, ginger, mace, and salt.

Alternate adding the dry ingredients and the cooled soaked oats to the butter mixture, mixing on low just until incorporated. Scrape down the bowl as needed and avoid overmixing.

Spread the batter evenly into the prepared pan.

Add the Crumble & Bake

Scatter the cold vanilla oat crumble evenly over the entire surface of the cake. Keep the crumble loose rather than pressing it into the batter. You want those irregular pieces to stay craggy and crisp as they bake.

Bake at 350°F (175°C) for about 40–50 minutes, or until the crumble is golden brown and a tester inserted into the center of the cake comes out clean or with just a few moist crumbs attached.

Allow the cake to cool in the pan before slicing.

Baker's Note: Don't Sleep on Mace

Mace comes from the same fruit as nutmeg, but its flavor is a little more delicate and aromatic. Here, it works beautifully with cinnamon and ginger, giving the oatmeal cake warmth and complexity without pushing it into aggressively spiced territory.

And while vanilla extract works beautifully in the cake itself, I love using an actual vanilla bean in the crumble. The combination of vanilla, butter, raw sugar, and toasted oats turns what could be a simple crumb topping into one of the best parts of the cake.