Cake

1. Preheat the oven to 350°F. Line an 8 x 8” baking pan with parchment paper. Set aside.
2. To a small bowl, add the chocolate chips and espresso powder. Add the hot water and cover with a piece of plastic wrap. Set aside.
3. In a medium bowl, whisk together the oil, yogurt, milk, eggs and vanilla until combined well. 
4. In a separate large bowl, whisk together the flour, sugar, cocoa powder, baking soda, baking powder and salt. 
5. Add the wet ingredients to the dry ingredients. Use an electric mixer to mix on low speed until just combined and then mix for 15 to 20 seconds more on medium-low speed.
6. Stir together the chocolate and hot water mixture until smooth. Add the melted chocolate mixture to the batter and mix on low speed for about 30 seconds until combined. Do not over mix. Use a large spatula to fold the batter together, scooping down to the bottom of the bowl to be sure all ingredients are incorporated. 
7. Transfer the batter to the prepared pan. Bake in the center rack of the oven for 26 to 28 minutes or until the edges are set and a toothpick inserted in the center of the cake comes out clean or with a few crumbs remaining. If the cake is getting too dark towards the end of the baking cycle, cover the top loosely with a piece of aluminum foil. Do not over bake. 
8. Remove the cake from the oven and place the pan on a wire rack to cool. Cool completely before frosting.

Chocolate Frosting

1. Melt the bittersweet chocolate over a double boiler, stirring often until melted and smooth. Remove from the heat.
2. Add the cocoa powder to the melted chocolate and stir to combine. Set aside to cool slightly.
3. To a large bowl, add the butter, vanilla and salt. Mix on low speed until light and creamy. 
4. Add the powdered sugar and mix on low speed until incorporated. If thick, add one tablespoon of milk or cream to help the frosting come together. Mix on medium speed for 2 minutes, stopping to scrape down the sides of the bowl several times.
5. Add the melted chocolate to the butter and sugar mixture and mix on low speed until combined. Increase speed to medium and mix for 30 seconds. Scrape down the sides of the bowl. 
6. If the frosting is too thick, add more milk or cream. If too thin, add more powdered sugar.
7. Use a large ice cream scoop or spoon to portion the frosting out on the top of the cooled cake. Using an offset spatula, spread the frosting evenly over the top of the cake, adding swirls as you work. 
8. Slice and serve!