Preheat the oven to 350°F.
Line the bottom of your cake pan with parchment paper and spray the sides with cooking spray.
Put the flour, walnuts, baking soda, salt, instant espresso powder, and unsweetened cocoa in a bowl and whish to mix.
In another bowl, mix together the brown sugar, water, vegan butter until the butter has melted.
Stir the wet ingredients into the dry ingredients. Whisk in the apple cider vinegar. you will see it react with the batter.
Pour the batter into the prepared pan and bake for 45 minutes. At 35 minutes check the cake by poking the centre with a knife. If it comes out clean it's done. If not, continue for the remaining 10 minutes.
When it's ready, the cake will be coming away from the edges of the pan and a cake tester will come out clean, apart from a few crumbs.
Once the cake is done, let it sit in its pan and cool completely.
Once completely cooled, run a knife around the cake and remove from the pan. Place on a plate or wire wrack to glaze.
Pour the glaze onto the centre of the cake and help it cover the cake with a offset spatula, pushing the glaze towards the edges.
Refrigerate for 30 minutes to let the glaze set.
Top with walnut halves.