Begin by heating your oven to 350°F (175°C). Grease two 9-inch cake pans and line the bottoms with parchment paper to prevent sticking.
In a large mixing bowl, combine the flour, sugar, cocoa powder, baking powder, baking soda, espresso powder, and salt. Whisk everything together until the mixture is evenly blended.
Add the eggs, milk, oil, and vanilla extract to the dry ingredients. Mix until the batter is smooth and fully combined.
Slowly pour in the boiling water while stirring gently. The batter will be quite thin, which is normal and helps keep the cake moist.
Divide the batter evenly between the prepared cake pans. Place them in the oven and bake for 30 to 35 minutes, or until a toothpick inserted into the center comes out clean.
Remove the cakes from the oven and let them cool in the pans for about 10 minutes. Carefully turn them out onto a wire rack and allow them to cool completely before frosting.
To make the blueberry frosting, beat the butter until soft and creamy. Gradually mix in the powdered sugar and ground freeze-dried blueberries. Add the cream, vanilla extract, and blueberry syrup if using, and continue beating until the frosting is light and fluffy.
Once the cakes are fully cooled, place one layer on a serving plate and spread an even layer of frosting on top. Set the second cake layer over it, then frost the top and sides evenly to finish.