Method

Preheat oven to 350°F. Line standard muffin tins with baking cups; set aside. Stir together cocoa powder, flour, sugar, baking soda, baking powder, and salt into a large bowl with a whisk. Add eggs, coffee, buttermilk, oil, and vanilla, and mix until smooth with an electric mixer, about 3 minutes. Scrape down the sides and bottom of bowl to assure batter is well mixed.

Fill each paper cupcake liner about ⅔ full. Bake approximately 20 minutes or until a toothpick inserted into the middle comes out clean. Cool in pan for 5 minutes, then transfer to a wire rack to cool completely.

Method Combine yolks, puree, and sugar in a small saucepan. Whisk to combine. Set over medium heat, and stir constantly with a wooden spoon, making sure to stir sides and bottom of pan. Cook until mixture is thick enough to coat back of wooden spoon, 5 to 7 minutes.

Remove saucepan from heat. Add butter, one piece at a time, stirring with the wooden spoon until consistency is smooth.

Transfer mixture to a medium bowl. Lay a sheet of plastic wrap directly on the surface of the curd to avoid a skin from forming; wrap tightly. Let cool; refrigerate until firm and chilled, at least 1 hour.

Method

Wipe the bowl of an electric mixer with paper towel and lemon juice, to remove any trace of grease.

Add egg whites and sugar, and simmer over a pot of water (not boiling), whisking constantly but gently, until temperature reaches 160°F, or if you don't have a candy thermometer, until the sugar has completely dissolved and the egg whites are hot.

With whisk attachment of mixer, begin to whip until the meringue is thick, glossy, and the bottom of the bowl feels neutral to the touch (this can take up to 10 minutes or so). *Don't begin adding butter until the bottom of the bowl feels neutral, and not warm.

Switch over to paddle attachment and, with mixer on low speed, add butter cubes, one at a time, until incorporated, and mix until it has reached a silky smooth texture (if curdles, keep mixing and it will come back to smooth). *If mixture is too runny, refrigerate for about 15 minutes and continue mixing with paddle attachment until it comes together.

Add vanilla and salt, continuing to beat on low speed until well combined.

Add blackberry puree a little at a time, beating between additions to incorporate fully. Don't worry if the mixture appears to separate, just keep beating it and the puree will incorporate.