Preheat oven to 350°F and spray a 9x5-inch loaf pan with baking spray. Coat inside of pan with flour.
In a medium or large bowl, whisk together flour, cocoa powder, sugar, baking powder, and salt. Add eggs and vanilla and whisk to combine. Add vegetable oil, milk, and instant coffee. Whisk until completely incorporated and instant coffee has dissolved.
Pour batter into the prepared pan and bake for 35 to 40 minutes, or until a toothpick inserted into the center of the cake comes out clean. (A few crumbs are okay!) Let the cake cool in the pan for 10 minutes, and then transfer to a wire rack to cool completely.
To make the glaze, dissolve instant coffee in hot water (or use strongly brewed and cooled coffee). Whisk together coffee, vanilla, and powdered sugar until completely incorporated. Glaze will be thick but should be easy to drizzle with a spoon. (If not, add ½ tbsp water until desired consistency is reached.) Drizzle glaze over cake and let it set up for 15 to 20 minutes. Cut into slices to serve.