For the frosting: Put the chocolate in a large, heatproof bowl. Bring the cream to a boil in a medium saucepan over medium heat. Pour the hot cream over the chocolate and let it sit, undisturbed, for 10 minutes, then add the salt and stir until melted and combined. Refrigerate, in the bowl, until thick and very cold, about 2 hours. Whip with an electric mixer on medium-high until fluffy, spreadable and slightly lightened in color.
For the cake: Preheat the oven to 350 degrees. Butter and flour two 9-inch cake pans. Line the bottom of each with parchment.
Combine the flour, cocoa powder, baking soda and baking powder in a medium bowl. Dissolve the instant espresso powder in 1 1/3 cups warm water.
Beat together the sugar, vanilla and eggs in a large bowl with an electric mixer on medium-high speed until light and fluffy, about 3 minutes. Beat in the mayonnaise and the melted chocolate until just combined.
Alternate beating the flour mixture and the espresso mixture on medium-high speed into the chocolate-egg mixture until just combined; work in 4 batches, beginning and ending with the flour mixture. Take care not to over mix.
Pour the cake batter into the prepared pans and bake until the cakes are set and the tops spring back when touched and a toothpick inserted into the center of the cakes comes out clean, about 30 minutes. Cool the cakes in the pans set on a rack for 20 minute. Invert the cakes onto the rack, remove the parchment paper and cool completely.
Frost the cake: Put 1 cake round on a cake stand or large serving plate and evenly spread 3/4 cup frosting over it. Top with the other cake round. Frost the top and sides with the remaining frosting. The cake can be frosted up to a day in advance. Let sit at room temperature for 30 minutes before serving.
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