Preheat the oven to 350°F (175°C) and grease two 8-inch round cake pans.
In a large mixing bowl, combine the flour, sugar, and baking powder.
Add the plant-based milk, vegetable oil, and cooled brewed coffee. Mix until just combined.
Fold the chopped walnuts gently into the batter.
Divide the batter evenly between the prepared cake pans.
Bake for 25–30 minutes, or until a toothpick inserted into the center comes out clean.
Let the cakes cool in the pans for 10 minutes before transferring them to a wire rack to cool completely.
Spread vegan coffee buttercream over one cake layer, then place the second layer on top.
Frost the top and sides with the remaining buttercream.
Decorate with walnut halves and a light dusting of cocoa powder or finely ground coffee, if desired.
Slice and serve with a hot cup of coffee or tea.