To begin your baking adventure, preheat your oven to 350°F (175°C). Prepare a 1½- to 2-quart baking dish by lightly coating it with cooking spray, ensuring a smooth release for your delicious cake.
In a spacious mixing bowl, combine ½ cup of whole-wheat pastry flour with ½ cup of all-purpose flour. Incorporate ⅓ cup of granulated sugar, followed by ¼ cup of sifted unsweetened cocoa powder. Finish by adding 1½ teaspoons of baking powder and ½ teaspoon of salt. Whisk these dry ingredients together thoroughly until they are well combined, forming the foundation of your cake batter.
In a separate glass measuring cup, whisk together 1 large egg, ½ cup of low-fat milk, 2 tablespoons of neutral oil (such as canola or avocado), and 2 teaspoons of vanilla extract. Gradually add this liquid mixture to your dry ingredients, stirring gently with a rubber spatula until just blended. If desired, fold in ¾ cup of semisweet chocolate chips to enhance the chocolatey goodness. Transfer the prepared batter into your baking dish, spreading it evenly.
In the same measuring cup, combine 1⅓ cups of hot brewed coffee with ⅔ cup of packed light brown sugar. Stir until the brown sugar completely dissolves, creating a sweet, aromatic coffee mixture. Carefully pour this liquid over the cake batter in the baking dish. Finally, sprinkle ¼ cup of chopped toasted walnuts or pecans over the top. Bake for 30 to 35 minutes, or until the top of the cake springs back lightly when touched. Allow the cake to cool for at least 10 minutes before serving, letting the magical sauce fully develop.