Preheat the oven to 375˚F (190˚C). Grease 9 x 13 inches baking pan with unsalted butter. Dust with flour to cover the pan and remove excess flour. 

In a large bowl, whisk together flour, sugar, baking soda, espresso powder, and salt. Set aside.

In a large saucepan, combine water, butter and chopped chocolate. Bring to a boil over medium-high heat, stirring occasionally. Remove from heat. Add the flour mixture and whisk to combine. Add the sour cream and mix well. Add eggs and whisk to combine.

Transfer the batter into the prepared baking pan. Bake in the oven for 20 – 25 minutes, or until the cake springs back when touch lightly. Remove from oven and cool completely.

When the cake is cooled, work on the buttercream. Set a medium heatproof bowl over a pot of simmering water. Make sure the water is not touching the bottom of the bowl. Add the chocolate into the bowl and let them sit untouched for 4 minutes. Stir until the chocolate is completely melted and smooth. Remove the bowl from heat and let the chocolate cool to nearly room temperature.

Meanwhile, in a large mixing bowl with an electric mixer fitted with a paddle attachment, beat the butter until smooth. Add sugar and beat on medium speed until pale and fluffy. Scrape down the sides of the bowl. Add extract, salt and 1/3 of the heavy cream. Mix on medium-low speed until just combined. Scrape down the side. Add half of the remaining cream. Mix at medium-low speed until smooth.

Remove the bowl from mixer. Use a spatula to make a large well in the mixture. Add the cooled chocolate into the well. Try to prevent the chocolate from touching the sides of the bowl. Fold in the chocolate until almost combined. Add the rest of the heavy cream. Mix with the spatula until combined.

Return the bowl to the electric mixer. Mix on medium-low speed until smooth and fluffy, about 30 seconds. Spread the buttercream over the cake. Buttercream will thicken up as it cools. Decorate with chocolate pearls, chocolate crisps, or chocolate shaving if desired.