Grease three 8-inch round cake pans and line the bottom with parchment paper.
Sift together the flour, cocoa, salt, baking powder and baking soda in a large bowl and set aside.
Pour the stout into a large measuring cup and mix in vanilla and espresso. Set aside.
In another large bowl, cream the butter and sugar together until it is completely mixed and light in texture. The beat in one egg at a time, combining thoroughly between each egg.
Add the mayonnaise and beat until creamy.
Mix together the dry ingredient with the stout and the butter mixture, adding a third of each to the mixing bowl until it is all combined together. Fold in the chocolate morsels so they are evenly distributed.
Pour the batter into the 3 cake pans, taking care to distribute as evenly as possible.
Bake for 35-40 minutes until a toothpick inserted into the center of each pan comes out clean. Remove the pans from the oven and allow to cool in the pan for 15-20 minutes.
Loosen the cake from the pans using a thin knife. Carefully turn out each layer onto a cooling rack, peel off the parchment paper, and allow to sit until the cakes are completely cool to the touch.
Meanwhile, make the frosting by beating the vanilla into the butter in a large bowl.
Add the cocoa powder, powdered sugar and milk gradually, stirring continuously until all ingredients are mixed in and the frosting has that fluffy consistency.
Frost each layer and then frost around the sides. Top with berries or sliced strawberries for a touch of color.