Preheat the oven to 350℉ & grease & parchment line 2-8" cake pans
Lightly toast the walnuts in a single layer on a sheet pan for about 5 minutes then when they are cooled grind in a food processor to small crumbs but not so much that it turns to paste! You can also chop them finely by hand
In a large mixing bowl combine the flour, baking powder, baking soda, salt & granulated sugar & whisk to distribute everything evenly or sift together
Measure the coffee, plant milk, vinegar & extract into a measuring cup then pour directly into the dry ingredients in the mixing bowl along with the melted butter & oil. Whisk to a smooth batter
Fold in the walnuts then divide the batter between your 2 prepared pans & bake for approximately 30-35 minutes or when they are springy to the touch when you gently press the centers or when a toothpick inserted comes out with moist crumbs not raw batter
Cool the cakes in the pans until you can safely touch them without burning yourself then flip them out onto a cooling rack & cool to cold
Meanwhile prepare the buttercream recipe by whipping the very slightly softened vegan butter until fluffy. Add the sifted confectioners sugar & mix smooth. Whip on high speed for 3 minutes until fluffy
Combine the cocoa powder, coffee granules & hot water & whisk smooth
When the coffee mixture is COLD add it to the buttercream & whip until evenly incorporated