Prep: Preheat oven to 350°F (175°C). Grease a 9 x 13-inch pan and line with parchment paper. Brew 1 cup of strong hot coffee and set aside.
Mix Dry Ingredients: In a large bowl, whisk together flour, cocoa powder, sugar, baking soda, baking powder, and salt until fully combined.
Add Wet Ingredients: Add eggs, buttermilk, oil, and vanilla. Beat with an electric mixer on medium speed for about 2 minutes until smooth.
Add Hot Coffee: Slowly pour in the hot coffee while mixing on low speed. The batter will be very thin and pourable - this is exactly right. A thin batter creates the fudgy texture.
Bake: Pour into the prepared pan and bake for 30-35 minutes, or until a toothpick comes out with a few moist crumbs. Cool completely on a wire rack before frosting.
Make the Ganache: Place chopped chocolate in a heatproof bowl. Heat the cream until it just starts to simmer. Pour over the chocolate and let it sit for 3 minutes. Stir from the center outward until smooth. Whisk in the butter and salt. Let it sit for 30-40 minutes until thick and spreadable.
Frost and Decorate: Spread the ganache evenly over the cooled cake. While still soft, press in the Mini Eggs (some whole, some crushed) for that signature Easter look. Slice and serve.