Make the cake

1. Preheat the oven to 350°F (175°C). Grease and flour a 9×13-inch pan (or line it with parchment) and set it aside.
2. In a large bowl whisk together the flour, cocoa, baking powder, baking soda, and salt until evenly combined.
3. In a separate mixing bowl, beat the oil, sugar, eggs, and vanilla on medium speed until smooth and well incorporated.
4. Reduce the mixer to low and add the buttermilk, then slowly add the dry ingredients. Scrape the bowl as needed so everything mixes evenly.
5. With the mixer still on low, gradually pour in the very hot coffee (or boiling water) and mix just until the batter is uniform. The batter will be thin — that’s normal.
6. Pour into the prepared pan and bake about 30–35 minutes, or until a toothpick inserted near the center comes out with a few moist crumbs (not wet batter).
7. Remove from the oven and allow the cake to cool completely on a wire rack before frosting.

Make the raspberry puree

1. Place the raspberries in a blender or food processor and puree until smooth. Press the puree through a fine-mesh sieve into a small saucepan, using the back of a spoon to work the juice through and leave the seeds behind.
2. Simmer the strained puree over medium heat until it reduces to a thick, concentrated syrup — roughly 10 minutes, ending with about ¼–⅓ cup of thick raspberry reduction. Let this cool fully in the refrigerator. The puree must be cold before it meets the buttercream.

Make the frosting

1. In a large bowl (or in the bowl of a stand mixer), beat the softened butter on medium speed until creamy and pale.
2. Gradually add 3 ½ cups powdered sugar on low, mixing until it’s incorporated and smooth.
3. Add ¼ cup of the chilled raspberry reduction and mix on low to combine. Then add heavy cream, one tablespoon at a time, until you reach your desired spreadable texture. If the frosting is still too loose, add up to an extra ½ cup powdered sugar. Beat briefly on medium speed just to smooth everything out.

Assemble

1. For cleaner frosting, pop the fully cooled cake into the freezer for 15–20 minutes to firm it up slightly — this cuts down on crumbs.
2. Spread the raspberry buttercream evenly over the top with an offset spatula or large knife. Slice and serve.