• Preheat oven to 350°F. Grease a 9 x 13-inch baking dish and line the bottom with parchment paper. For the most even bake, use a light-colored metal baking pan.
• In a large mixing bowl, whisk together the flour, sugars, cocoa powder, baking soda, baking powder, and salt.
• Add the buttermilk, oil, sour cream, eggs, and vanilla bean paste. Whisk until mostly combined, scraping down the sides of the bowl as needed.
• Gradually pour in the hot coffee while whisking until the batter is smooth. The batter will be thin. Do not overmix.
• Bake for 30-35 minutes or until the center springs back when lightly touched, and a toothpick inserted comes out with a few moist crumbs.
• Let the cake cool completely to room temperature. For cleaner cuts, place the cooled cake in the freezer for 20-30 minutes before assembling.

For the Frosting

• Place the softened butter in a large mixing bowl and beat with electric mixers until smooth, creamy, and pale in color.
• Gradually add the powdered sugar and mix on low speed, then increase speed to medium and beat until light and fluffy.
• Add the heavy cream, vanilla bean paste, and salt. Beat until smooth, then mix on medium-high speed for 2 minutes until light and airy.

For Assembly

• Lightly grease a cookie cutter that is slightly smaller than your serving cups, then cut rounds from the chilled cake. Cutting the cakes can be messy. Place one round into each cup and gently press into the bottom, leaving about ¼-½ inch below the rim for the frosting.
• Pipe a layer of frosting to the rim of the cup. Smooth the tops with an offset spatula or butter knife.
• Carefully pour the nonpareils in a shallow bowl. Invert each cake and place the frosting side down into the nonpareils until completely coated.