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Brown Sugar and Oat Gooey Butter Cake

Prep Time 15 minutes
Cook Time 48 minutes
Cooling Time 2 hours
Total Time 3 hours
Servings: 1 8x8-inch baking dish
Course: Dessert
Cuisine: American

Ingredients
  

Oats (for entire recipe)
  • 70 g old-fashioned rolled oats about ¾ cup
  • → Blend into a fine oat flour then divide:
  • 40 g for the base
  • 30 g for the topping
Brown Sugar Oat Base
  • 115 g unsalted butter melted (½ cup)
  • 120 g dark brown sugar packed (½ cup)
  • 1 large egg
  • 1 tsp vanilla extract
  • 120 g all-purpose flour 1 cup
  • 40 g oat flour from above
  • ½ tsp fine sea salt
Brown Sugar Gooey Oat Topping
  • 170 g unsalted butter melted (¾ cup)
  • 300 g dark brown sugar packed (1½ cups)
  • 2 large eggs
  • 1 large egg yolk
  • 60 g heavy cream ¼ cup
  • 60 g all-purpose flour ½ cup
  • 30 g oat flour from above
  • 1 tsp vanilla extract
  • ½ tsp fine sea salt
For Finishing
  • Powdered sugar for dusting
  • Vanilla ice cream
  • Caramel sauce store-bought or homemade

Equipment

  • 8x8-inch light metal baking pan
  • Parchment paper
  • Mixing bowls
  • Rubber spatula
  • Whisk
  • Digital kitchen scale
  • Small blender or food processor
  • Ice cream scoop

Method
 

Prep
  1. Preheat oven to 325°F (163°C).
  2. Line an 8×8-inch pan with parchment paper.
  3. Blend 70 g rolled oats (about ¾ cup) into a fine flour.
  4. Divide into 40 g for the base (about ½ cup) and 30 g for the topping (about ¼ cup).
Make the Base
  1. In a bowl, whisk together melted butter (115 g / ½ cup) and brown sugar (120 g / ½ cup packed) until smooth.
  2. Whisk in 1 egg and vanilla (1 tsp).
  3. Add flour (120 g / 1 cup), oat flour (40 g / ½ cup), and salt (½ tsp).
  4. Stir until a thick dough forms.
  5. Press evenly into the prepared pan.
Par-Bake
  1. Bake for 10 minutes, until just lightly set.
  2. It should still look pale and slightly underbaked.
Make the Gooey Topping
  1. While the base bakes, prepare the topping.
  2. In a bowl, whisk melted butter (170 g / ¾ cup) and brown sugar (300 g / 1½ cups packed) until smooth.
  3. Whisk in 2 eggs + 1 egg yolk until fully combined.
  4. Add heavy cream (60 g / ¼ cup) and vanilla (1 tsp) and whisk again.
  5. Stir in flour (60 g / ½ cup), oat flour (30 g / ¼ cup), and salt (½ tsp) until smooth.
Assemble
  1. Pour the gooey topping over the warm base.
  2. Do not spread — it will settle into an even layer on its own.
Bake
  1. Bake at 325°F for 32–40 minutes.
  2. The cake is done when: edges are set, top is lightly crackled, and the center still jiggles slightly,
  3. Do not overbake.
Cool
  1. Cool at room temperature for at least 2 hours before slicing.
  2. The center will continue to set into a soft, custardy texture.
Finish & Serve
  1. Dust generously with powdered sugar.
  2. Serve slightly warm with vanilla ice cream and a drizzle of caramel sauce.

Notes

• The center will look underbaked when you remove it from the oven — this is correct. It will continue to set as it cools into a soft, custardy texture.
• Do not overbake. If the center no longer jiggles at all in the oven, it is likely overbaked and will lose the gooey texture.
• For best results, use a light-colored metal pan. Glass or dark pans may require adjusting bake time and can lead to overbaking.
• Blending the oats into a fine flour creates a smooth, cohesive texture while still giving that subtle oat flavor.
• For clean slices, allow the cake to cool completely before cutting. For a softer, gooier texture, serve slightly warm.

Storage

Room Temperature:
Store covered at room temperature for up to 2 days.
Refrigerator:
Store in an airtight container in the fridge for up to 5 days.
Reheating:
Warm individual slices in the microwave for 10–15 seconds to bring back the gooey texture.