Make the cakes. In a large bowl, beat the margarine, sugar, yogurt, flour, baking powder and ground almonds for 2-3 minutes until it is smooth and glossy. Split the batter equally into two bowls.
Add the espresso, chopped walnuts and molasses into half of the batter, and add the 1 ½ tsp soy milk into the other half of the batter. Mix just to combine, then pour into 2 greased 8×4" cake pans.
Bake at 325F for 30-35 mins until cooked, cool in the pans for 5 minutes, then turn the cakes out and cool completely before assembling.
Make the buttercream. In a large bowl, using an electric mixer on medium-high, whip the vegan butter and cocoa. Add in half of the icing sugar, followed by half the soy milk, and mix again. Add the remaining icing sugar and soy milk, plus the vanilla, and mix again. If it seems too dry, add 1 tsp more soy milk at a time until it’s creamy and thick enouch for piping. You don’t want it to be too runny or dripping off a spoon.
Assemble. Trim the edges off the cooled cakes, then cut into 4 equal strips. Lay one vanilla and one coffee-walnut strip next to each other, using some buttercream to stick them together. Spread a bit more buttercream on the top. Stick the remaining two strips together with buttercream and place them on top to create a checkerboard. Spread a bit more buttercream over the top of the cake.
Decorate. Roll the marzipan out on a work surface dusted with icing sugar. Make it into a rectangle as long as the cake and wide enough to wrap around the cake. Lay the buttercream side of the cake down on the marzipan and spread buttercream over the remaining three sides of the cake (not the ends). Roll the cake in the marzipan, pressing it tight, and brush the join with water, pressing to seal. Turn the cake over so that the join is underneath. Trim the ends of the cake to show the checkerboard. Smooth the marzipan with your hands and pinch the edges if you like. Stick the walnut pieces decoratively on top of the cake with little dots of buttercream to secure it. Slice and enjoy!