Instant Espresso Powder (2 ½ tbsp): The star ingredient. You must use espresso powder, not standard instant coffee granules. Espresso powder is much more concentrated and dissolves seamlessly to provide a deep, dark roast flavor without adding excess liquid to the batter.Hot Whole Milk (3 tbsp): Used specifically to "bloom" the espresso powder, unlocking its full aromatic potential before it hits the flour.All-Purpose Flour (2 cups): Provides the necessary structural backbone to support the heavy chopped nuts without making the crumb too dense.Light Brown Sugar (1 ½ cups): The molasses in the brown sugar pairs perfectly with the bitterness of the coffee, adding immense moisture and deep caramel notes that white sugar simply cannot provide.Unsalted Butter (1 cup, softened): Using unsalted butter allows you to control the exact sodium levels. It must be at room temperature to trap air during the creaming process.Large Eggs (4 whole, room temperature): Act as the primary binding agent, emulsifying the butter and providing lift and structure.Chopped Walnuts (1 ½ cups, divided): Provide a necessary textural contrast. You will fold 1 cup into the batter and save the remaining half-cup for garnishing the frosting.Baking Powder (2 tsp) & Baking Soda (½ tsp): The leavening system required to lift the batter and create a light, airy sponge.Pure Vanilla Extract (1 tbsp): Adds sweet floral notes that round out the sharp, bitter edges of the espresso.Fine Sea Salt (¾ tsp): A critical flavor enhancer that cuts through the sweetness and elevates the roasted nut profile.For the Espresso Mascarpone Frosting: 8 oz mascarpone cheese (chilled), ½ cup unsalted butter (softened), 3 cups powdered sugar, 1 tbsp espresso powder dissolved in 1 tbsp heavy cream.