Cake Layers:
Preheat your oven to 350°F (175°C). Butter four 8-inch cake pans and line with parchment paper.
In a large bowl, whisk together flour, sugar, cocoa, baking powder, baking soda, salt, and espresso powder.
Add milk, oil, eggs, and vanilla to the flour mixture and mix on medium speed until well combined. Carefully add boiling water to the cake batter until well combined.
Fold in chopped blackberries gently.
Distribute cake batter evenly between the prepared cake pans. Bake for 30-35 minutes, until a toothpick inserted in the center comes out clean.
Remove from the oven and let cool for about 10 minutes, then remove from the pan and cool completely.
Blackberry Frosting:
Beat the butter until light and fluffy. Gradually add powdered sugar, one cup at a time, beating well on medium speed.
Stir in blackberry puree, vanilla, salt, and food coloring until fully combined. Adjust consistency with more powdered sugar or puree if necessary.
Glossy Ganache:
Heat the heavy cream in a saucepan over medium heat just until it begins to simmer; do not let it come to a boil.
Place the finely chopped chocolate in a heat-proof bowl.
Pour the hot cream over the chocolate, letting it sit for 1-2 minutes to gently melt the chocolate.
Add the corn syrup or honey and vanilla extract, if using.
Stir the mixture slowly with a spatula, starting from the center and working outward, until the ganache is smooth and shiny.
Let it cool slightly before pouring over your cake for that lustrous, mirror-like finish.
Assembly:
Place one layer of cake on a plate or cake stand. Spread a layer of blackberry frosting over the top. Repeat with the remaining layers. Frost sides of the cake as well.
Pour the chocolate ganache over the top of the cake.
Decorate with whole blackberries.