Preheat oven to 350°F. Line a 8" square pan with parchment that has an overhang for easy removal from the pan. Lightly grease with cooking spray.
In a small bowl, whisk water and ground flaxseeds, set aside until thick.
In a large bowl, combine flour, sugar, cocoa powder, baking powder, salt and espresso powder.
In a measuring cup, whisk together the almond milk, oil, vanilla extract and flax seed mixture. Add to the flour mixture, along with the chocolate chips and walnuts, stir until just combined. Pour the batter into the prepared baking pan and smooth the top.
Bake for 45~55 minutes, or until a toothpick inserted into the center comes out clean.
Let cool for 20~30 minutes before cutting into squares and serving. Store leftovers in an airtight container for up to 4 days.