• Preheat the oven to 350°F (175°C). Lightly grease an 8-inch square pan and line it with parchment paper. For a thiner cake, use a 9-inch square pan.
• In a large bowl, whisk together the eggs, milk, oil, and vanilla extract until well combined.
• Add both the sugar and brown sugar to the mixture, whisking well.
• Mix in the flour, cocoa powder, baking powder, baking soda, salt, and espresso powder (if using). Whisk until the batter is smooth with no visible clumps.
• Pour the batter into the prepared pan. Bake for 30-35 minutes. The cake is done when a toothpick inserted into the center comes out with a few crumbs. Avoid over-baking.
• Remove from the oven and let the cake cool in the pan for 10 minutes. Use the parchment paper to lift the cake out and transfer it to a wire rack to cool completely.

Chocolate Buttercream

• Melt the chocolate using a double boiler or in a microwave-safe bowl. Allow it to cool for 5 minutes.
• In a separate bowl, beat the room-temperature butter, powdered sugar, and salt together until smooth.
• Mix in the melted and cooled chocolate.
• Dissolve the cocoa powder in milk, and then add it to the butter mixture.
• Add the heavy cream and vanilla extract, beating until the frosting achieves a smooth consistency.

Assemble

• Once the cake is completely cooled, spread a generous amount of frosting over the top using a spatula. If desired, sprinkle the frosted cake with decorative sprinkles. Slice, serve, and enjoy!