Preheat the oven to 350ºF / 175ºC and spray a half sheet pan (13x18 inches) with nonstick cooking spray.
Combine the soy milk and vinegar in a measuring cup. Stir then set aside while you prepare the remaining ingredients.
Add the flour, sugar, cocoa powder, baking powder, baking soda and salt to a large mixing bowl. Whisk to combine.
Add the melted butter, vanilla and buttermilk mixture. Whisk together until the dry ingredients are incorporated.
Add the hot water and stir again until a smooth batter forms.
Spread the cake batter into your prepared pan. Use an offset spatula to spread it into an even layer.
Bake in the preheated oven for 20-25 minutes until the center is set and a toothpick inserted in the center comes out clean.
As soon as you remove the cake from the oven, prepare the frosting.
Melt the butter in a saucepan over medium heat.
Add the cocoa powder, soy milk and vanilla. Whisk to combine.
Remove from heat and stir in the powdered sugar and chopped pecans.
Spread over the hot cake in an even layer. Allow the cake and frosting to cool completely before slicing.