Brown Butter Coffee Walnut Sheet Cake with Coffee Fudge Frosting

By Sprig Originals ✓ · cooked up in Sprig

A one-pan take on the British teatime classic: a moist brown-butter cake built on ground toasted walnuts and strong espresso, brushed with a coffee syrup while warm, then covered in an old-fashioned cooked coffee fudge frosting that sets to a soft, fudgy sheen. Buttery maple-candied walnuts on top give every square a crunch. No layers to level, no crumb coat — just a deep, grown-up coffee cake you slice straight from the pan.

Ingredients

• 3/4 cup unsalted butter
• 1 1/4 cups walnut halves (for grinding into the batter)
• 2 cups all-purpose flour
• 1 1/2 tsp baking powder
• 1/2 tsp baking soda
• 3/4 tsp fine sea salt
• 2 tbsp instant espresso powder
• 1/3 cup boiling water
• 1 cup packed light brown sugar
• 1/2 cup granulated sugar
• 3 large eggs, room temperature
• 1 large egg yolk, room temperature
• 2 tsp vanilla extract
• 3/4 cup sour cream, room temperature

• 1 tbsp instant espresso powder
• 1/4 cup hot water
• 2 tbsp packed light brown sugar

• 1 cup walnut halves
• 1 tbsp unsalted butter
• 2 tbsp maple syrup
• 1/4 tsp flaky sea salt

• 1 cup packed dark brown sugar
• 6 tbsp unsalted butter
• 1/4 cup heavy cream
• 2 tbsp strong brewed coffee or espresso, hot
• 1 1/2 tsp instant espresso powder
• 1/4 tsp fine sea salt
• 1 3/4 cups powdered sugar, sifted
• 1/2 tsp vanilla extract

Instructions

1. Heat the oven to 350°F. Spread all 2 1/4 cups walnuts (the 1 1/4 cups for the batter plus the 1 cup for candying) on a rimmed baking sheet and toast 7 to 9 minutes, until fragrant and one shade darker. Cool completely, then set aside 1 cup of the prettiest halves for the topping.
2. Butter a 9x13-inch metal baking pan and line the bottom with parchment. Melt the 3/4 cup butter in a light-colored saucepan over medium heat, then keep cooking, swirling often, 5 to 7 minutes, until the milk solids at the bottom are deep amber and it smells nutty. Scrape every bit into a large bowl, including the browned specks, and let cool 10 minutes.
3. In a food processor, pulse the 1 1/4 cups cooled toasted walnuts with 1/4 cup of the flour in short bursts until they look like coarse sand — stop before it turns to paste. Whisk this into the remaining 1 3/4 cups flour along with the baking powder, baking soda, and 3/4 tsp salt.
4. Stir 2 tbsp espresso powder into 1/3 cup boiling water until dissolved; let cool to warm. Whisk the brown sugar and granulated sugar into the brown butter until smooth and glossy, then whisk in the eggs and yolk one at a time, followed by the vanilla, the dissolved espresso, and the sour cream. The batter should be smooth and coffee-colored.
5. Add the dry ingredients and fold with a spatula just until no streaks of flour remain, about 20 strokes. Scrape into the prepared pan and smooth the top.
6. Bake 32 to 38 minutes, until the cake springs back when pressed in the center and a toothpick comes out with a few moist crumbs (internal temperature 205 to 210°F). Set the pan on a rack.
7. While the cake bakes, make the soak: stir 1 tbsp espresso powder and 2 tbsp brown sugar into 1/4 cup hot water until dissolved. As soon as the cake comes out, poke it all over with a toothpick and brush the entire surface with the coffee soak. Let the cake cool completely in the pan, at least 1 1/2 hours.
8. Candy the walnuts: melt 1 tbsp butter with the maple syrup in a skillet over medium heat, add the reserved 1 cup walnut halves, and toss constantly 3 to 4 minutes until the syrup thickens and coats the nuts. Spread on parchment, sprinkle with flaky salt, and cool completely, then break apart.
9. Make the frosting: combine the dark brown sugar, 6 tbsp butter, heavy cream, hot coffee, 1 1/2 tsp espresso powder, and 1/4 tsp salt in a medium saucepan. Cook over medium heat, stirring, until the butter melts and the sugar dissolves, then bring to a full boil and boil exactly 2 minutes without stirring. Remove from the heat and let cool 15 minutes, to about 110°F.
10. Beat the sifted powdered sugar and vanilla into the warm coffee mixture with a hand mixer on medium, 1 to 2 minutes, until smooth, thick, and pourable like warm fudge sauce. If it is stiff, beat in hot coffee 1 tsp at a time; if loose, beat in more powdered sugar 2 tbsp at a time.
11. Working quickly, pour the frosting over the cooled cake and spread to the edges with an offset spatula — it starts to set within a few minutes. Immediately scatter the candied walnuts over the top and press them in lightly. Let stand 30 minutes for the frosting to firm, then cut into 12 squares.

Chef's tips

Nutrition (per serving, estimated)

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