Position a rack in the center of the oven and heat it to 350°F. Grease three 8-inch round cake pans with the 14 g softened butter, line the bottoms with parchment rounds, and lightly grease the parchment. Set the pans on a digital scale one at a time so the batter can be divided evenly later. Confirm that the eggs and buttermilk are near room temperature, which helps the batter emulsify instead of becoming greasy or curdled.

Set a fine-mesh sieve over a large mixing bowl. Add the flour, cocoa powder, granulated sugar, baking powder, baking soda, and salt, then sift everything into the bowl. Whisk for about 30 seconds, reaching across the bottom and sides, until the mixture is one uniform cocoa color with no pale flour streaks. Thorough distribution matters because the thin batter will not receive prolonged mixing after the liquids are introduced.

In a medium bowl, whisk the eggs until the whites and yolks are fully blended, then whisk in the buttermilk, oil, and vanilla. Pour this mixture into the dry ingredients and whisk from the center outward just until no dry flour remains. Slowly stream in the hot coffee while whisking gently. Scrape the bowl with a spatula; the finished batter should be smooth, glossy, and quite fluid, with no cocoa pockets, but it should not be beaten after it becomes uniform.

Divide the batter equally among the prepared pans, using about 520 g per pan, and tap each pan lightly on the counter to release large air bubbles. Bake all three on the center rack for 24 to 28 minutes, rotating their positions after 18 minutes only if the oven browns unevenly. The layers are done when their centers spring back, the edges barely pull from the pans, and a tester shows moist crumbs but no wet batter.

Place the pans on wire racks for 15 minutes so the delicate crumb can set. Run a thin spatula around each edge, invert the cakes onto the racks, remove the parchment, and turn them upright. Cool completely for about 75 minutes before trimming or frosting. If the tops are domed, use a long serrated knife to remove only the raised portion, keeping the blade parallel to the counter. Warm cake will melt the peanut butter frosting and weaken the layers.

Using a stand mixer fitted with the paddle, beat the 454 g softened butter on medium speed for 2 minutes until creamy and slightly lighter. Add the peanut butter and beat for 1 minute, scraping the bowl. Mix in the confectioners’ sugar on low in three additions, followed by the cream, vanilla, and salt. Beat on medium for 2 minutes. The frosting should hold a curved peak and spread without tearing; mix briefly by hand to remove large air pockets.

Place the finely chopped semisweet chocolate in a heatproof bowl. Heat the 120 g cream in a small saucepan until steaming with bubbles around the edge, then pour it over the chocolate. Leave it undisturbed for 2 minutes before stirring from the center outward until glossy. Cool for 15 to 25 minutes, stirring occasionally, until it falls from a spoon in a slow ribbon. Meanwhile, reserve about 120 g whole peanut butter cups and coarsely chop the remaining 180 g.

Secure one cake layer to an 8-inch cake board or serving plate with a dab of frosting. Spread about 225 g frosting evenly over it, stopping 1/4 inch from the edge, and scatter with half the chopped peanut butter cups. Repeat with the second layer, frosting, and remaining chopped candy, then add the final layer upside down. Apply about 250 g frosting in a thin crumb coat over the top and sides. Refrigerate uncovered for 30 minutes until the coating feels firm.

Reserve about 300 g frosting in a piping bag fitted with an open-star tip. Cover the chilled cake with the remaining frosting, smoothing it with an offset spatula and icing scraper. Test ganache on the back edge; it should form a slow drip and stop before the board. Spoon drips around the rim, spread the remaining ganache across the top, and chill for 20 minutes. Pipe frosting swirls, add the reserved peanut butter cups, then cut into 24 slices with a hot, dry knife.