• Make the cake: 
• Preheat the oven to 350°F. Grease a 9-by-13-inch glass or metal cake pan with the 1 tablespoon of room temperature butter.
• Whisk together the flour, cocoa, baking powder, baking soda, and salt in a large bowl. Combine the buttermilk and vanilla in a medium bowl or a 2-cup liquid measuring cup.
• In the bowl of a stand mixer fitted with the paddle attachment (or in a large bowl, if using a handheld mixer), cream the 1 cup of butter and the brown sugar on     low speed until creamy and well combined. Increase the speed to     medium-high and beat until light and airy, about 2 minutes. Reduce the mixer speed to medium-low and add the eggs one at a time, mixing well  between each addition and scraping down the side and bottom of the bowl as needed. Once all 3 eggs are added, beat for 1 minute on medium speed to  get the mixture nice and fluffy.
• Reduce the speed to medium-low and add the flour mixture alternately with the buttermilk mixture in three batches, starting with the flour. Add the melted     chocolate and mix on medium speed until well incorporated, stopping the mixer to scrape down the side and bottom of the bowl as needed.
• Use a  rubber or offset spatula to scrape the batter into the prepared pan and  even it out as much as possible. Bake until a cake tester inserted into the center of the cake comes out clean and the center of the cake resists light pressure, about 40 minutes. Remove from the oven and set aside to cool completely, at least 2 hours, before frosting.
•  Make the frosting:
• ‍ Put the butter in the bowl of a stand mixer fitted with the whisk attachment (or in a medium bowl, if using a handheld mixer). Beat on medium-high speed until smooth. Turn off the mixer and sift the confectioners’ sugar into the bowl and combine on low speed. Add the melted chocolate, soy sauce, and vanilla and beat on low speed until combined. Increase the mixer speed to medium-low and whip until glossy, 15 to 30 seconds.
• Use an offset spatula or butter knife to spread the frosting on top of the cooled cake. Cut into squares and serve.
• DO AHEAD:
• The cake will keep, loosely covered with plastic wrap in the refrigerator, for up to 3 days. Let it sit out at room temperature for15 to 20 minutes before slicing and serving.

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