Preheat the oven to 350°F. Grease and line a 9x9-inch baking pan with parchment paper.
Pulse the walnuts in a blender or food processor until they are a coarse, flour-like texture. You should have about 1 3/4 cups.
In a bowl, whisk together the eggs, oil, and vanilla.
In another bowl, mix the ground walnuts, flour, monk fruit sweetener, espresso powder, baking powder, and salt.
Add the wet ingredients to the dry ingredients and stir until just combined.
Pour the batter into the prepared pan and spread it evenly.
Bake for 28 minutes at 350°F. A toothpick inserted in the center might come out with a few crumbs, but that's ok. The cake will cook a bit as it cools, and baking it longer makes it dry.
Cool in the pan, then frost with coconut whipped topping mixed with a pinch of espresso powder, and top with chopped walnuts. Cut into 9 squares.