Chocolate-Covered Cake with Creamy Filling

Instructions

For the Cake:

1. Line an 8-inch (20cm) square baking pan with parchment paper. Set it aside. 
2. Preheat the oven to 350°F (180°C).
3. In a large bowl, combine milk and vegetable oil. Microwave for 40-60 seconds until warm.
4. Add cocoa powder and stir until fully dissolved. Add yogurt, sugar, and vanilla. Stir for a few seconds to combine. 
5. Add the egg and mix well using a wire whisk. 
6. Sift in flour, baking powder, baking soda, and salt. Gently fold until just combined. Don't overmix the batter. 
7. Pour the batter into the prepared pan and bake for 15-18 minutes or until a toothpick inserted into the center comes out clean. Let it cool at room temperature.

For the Filling:

1. Combine semi-sweet chocolate and heavy cream in a bowl. Melt using a double boiler or microwave in 30-second intervals, stirring in between. Let it cool to room temperature for a few minutes.
2. Note: For milk chocolate (25-30% cocoa solids), use only 1/4 cup of heavy cream. Otherwise, the filling will be too soft.
3. Combine the cream cheese, powdered sugar, and vanilla in another bowl. Stir with a spatula until creamy. You can use an electric mixer if your cream cheese is too lumpy.
4. Add the cooled chocolate ganache and stir until smooth.
5. Refrigerate the filling for 30-40 minutes if necessary, stirring occasionally until thick enough to spread and support the cake. 

Assembling the Cake:

1. Cut the cake into three equal strips. Place the first layer on a plate or a wire rack (if it can fit in your freezer).
2. Using a piping bag (optional), spread the filling over the cake. Place another cake layer on top, more filling, and finish with the third cake layer.
3. If necessary, spread some of the filling on the sides of the cake to smooth it out and cover any imperfections.
4. Place the cake in the freezer for about one hour.

For the Frosting:

1. In a bowl, combine semi-sweet chocolate and butter. Melt using a double boiler or microwave in 30-second intervals, stirring until smooth.
2. Add finely chopped walnuts and mix.
3. Remove the cake from the freezer and place it on a wire rack. Pour the frosting gradually, spreading it with a spatula to ensure the cake will be completely covered with chocolate.
4. For the chocolate to stick and start to firm up when it comes in contact with the cake, it is important to ensure that the cake is well chilled.
5. Using two spatulas, transfer the cake to the serving plate. Keep the cake in the fridge until ready to serve or for up to 3 days.