Step 1: Bake the Chocolate Cake

1. Preheat oven to 350°F (175°C). Line a 9×13-inch baking pan with parchment paper.
2. In a large bowl, whisk together flour, cocoa powder, sugar, baking soda, baking powder, and salt.
3. Add egg, buttermilk, oil, and vanilla. Mix until combined.
4. Stir in hot water or coffee (the batter will be thin—this is normal).
5. Pour batter into the prepared pan and bake for 20-25 minutes, or until a toothpick inserted in the center comes out clean.
6. Let the cake cool completely before cutting.

Step 2: Make the Strawberry Filling

1. In a small saucepan over medium heat, combine strawberries, sugar, and lemon juice.
2. Cook for about 5 minutes, stirring occasionally, until the strawberries soften and release their juices.
3. Mix cornstarch with water and stir it into the strawberry mixture. Cook for another 1-2 minutes until thickened.
4. Remove from heat and let cool completely.

Step 3: Prepare the Chocolate Ganache

1. Heat heavy cream in a small saucepan over medium heat until it begins to simmer.
2. Remove from heat and pour over the chopped chocolate in a heatproof bowl. Let sit for 1 minute, then stir until smooth.
3. Stir in vanilla extract. Let cool slightly before using.

Step 4: Assemble the Mini Cakes

1. Use a round cookie cutter (about 2-3 inches wide) to cut circles from the cooled cake.
2. Spread a layer of strawberry filling onto one cake round, then top with another cake round.
3. Spoon chocolate ganache over each mini cake, letting it drip down the sides.
4. Garnish with fresh strawberries and a drizzle of white chocolate if desired.