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1
Stir espresso powder into boiling water; reserve.
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2
In medium bowl, whisk together cocoa, sugar, and brown sugar; reserve.
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3
Adjust oven rack to middle position and preheat oven to 350u00b0F. Lightly coat 8-inch square cake pan with baking spray.
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4
Combine sugar, corn syrup, and water in medium heavy-bottomed saucepan. Cook over medium-high heat until the sugar is dissolved. Dampen clean pastry brush with water and wipe down any sugar crystals from sides of pan. Boil mixture, swirling (but not stirring) pan occasionally, until sugar turns deep amber, 5 to 7 minutes. Immediately remove from heat and add cream; mixture will bubble and sputter. Return to heat and stir over low heat until mixture is smooth. Set aside.
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5
In small or medium stainless steel skillet (See Notes), melt butter over medium heat. Cook butter, swirling pan occasionally, until beginning to foam. As foam settles, brown bits of butter will become visible at the bottom of the pan (5 to 7 minutes). Immediately remove from heat and stir into reserved caramel.
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6
In large bowl, whisk together flour, baking powder, and salt. In medium bowl, whisk together brown sugar, egg yolks, and vanilla until combined. Whisk in milk and caramel-brown butter mixture.
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7
Slowly whisk wet mixture into dry mixture, mixing only until combined. With rubber spatula, scrape batter into prepared pan.
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8
Sprinkle batter evenly with reserved cocoa-sugar mixture, then slowly and evenly pour coffee over powder mixture. Bake until sides of cake pull away from sides of pan, 25 to 30 minutes. Transfer to cooling rack and cool 30 minutes prior to serving (it will still be warm). Sprinkle with Maldon salt, if desired, and serve with vanilla ice cream.
