Preheat oven to 350°F (180°C).

For cake: In the bowl of a stand mixer fitted with the paddle attachment, beat granulated sugar, brown sugar, oil, and eggs at medium speed for 2 to 3 minutes. Add sour cream and vanilla, and beat until smooth. Add flour, cocoa, baking powder, baking soda, and kosher salt. Fold until combined.

In a small microwave-safe bowl, heat milk on high for 45 seconds. Add espresso powder, and stir until combined. Add to batter, and stir until fully combined.

Spray a 9-inch round cake pan with baking spray with flour. Pour batter into prepared pan.

Bake until a wooden pick inserted in center comes out clean, 40 to 50 minutes. Let cool in pan for 5 minutes. Remove from pan, and let cool completely on a wire rack.

For buttercream:

In the bowl of a stand mixer fitted with the paddle attachment, beat butter and confectioners’ sugar at medium speed until combined.

In a small microwave-safe bowl, heat chocolate chips and cream on high for 40 seconds. Stir until smooth. (If needed, heat in 15-second intervals until all chocolate is melted and mixture comes together smoothly.)

With mixer on medium speed, slowly add chocolate mixture to butter mixture, beating until fluffy and smooth. (If buttercream seems too loose, refrigerate for 15 minutes and then beat again.) Beat in vanilla and salt. Spread buttercream on top of cooled cake. Garnish with chocolate curls and sea salt, if desired. Refrigerate in an airtight container for up to 4 days.