Preheat oven to 350°F (175°C). Grease and line two 8-inch cake pans (or one 8×3-inch or 9-inch pan) with parchment paper.
In a large bowl, whisk together cake flour, black cocoa powder, baking powder, and sea salt until evenly combined.
In a separate bowl, whisk together granulated sugar, oil, eggs, and sour cream until smooth.
Sift the dry ingredients into the wet ingredients and whisk until partially combined.
Slowly pour in the hot coffee and continue whisking until the batter is smooth and fully incorporated.
Divide the batter between prepared pans and smooth the tops.
Bake for approximately 25 minutes for two pans, or 40 minutes if using one tall pan, until the center springs back when lightly pressed.
Allow cakes to cool in the pans for 10 minutes. Turn out onto a clean towel or wire rack and cool completely.
If desired, trim domed tops and drizzle cooled espresso over the cake layers as a soak.
For the frosting, beat cold mascarpone and powdered sugar until smooth.
Add vanilla bean paste and mix on medium speed.
Gradually stream in the cold heavy cream in several additions, scraping down the bowl as needed.
Increase mixer speed and whip until stiff peaks form. Avoid overmixing.
If using espresso, slowly stream it into the frosting while mixing on low speed until just combined.
Spread frosting generously over the cooled cake.
Finish with a generous dusting of cocoa powder before serving.