Preheat your oven to 180°C (356°F) and grease a baking pan. In an electric mixer, beat the eggs until they are thick and pale. Gradually add sugar while continuing to mix, achieving a fluffy consistency. Gently fold in a sifted mixture of all-purpose flour and unsweetened cocoa powder to maintain airiness. Pour the batter into the prepared pan and bake for 15 minutes, ensuring not to open the oven door for the first 10 minutes. Let cool for at least 8 hours or overnight for the best texture.

In a bowl, whisk together cornstarch and cocoa with a portion of the milk. In a saucepan, heat the remaining milk with sugar and vanilla extract until just before boiling. Remove from heat and gradually whisk in the cocoa mixture until it thickens, which should take about 5 minutes. Cover the surface with plastic wrap as it cools to maintain a smooth custard.

In a mixing bowl, beat the heavy whipping cream and sugar until it achieves soft peaks. Gradually sprinkle in cocoa powder towards the end of whipping for an even chocolate flavor. Chill the whipped cream until ready to use.

Once the cake has completely cooled, carefully slice it in half horizontally using a serrated knife. Place the bottom half on a serving plate and spread a generous layer of the custard cream over it. Gently place the second half of the cake on top, followed by the remaining custard cream spread evenly over the top. Spread the chilled whipped cream over the assembled cake for a delightful finish.

In a small saucepan over low heat, melt the butter along with unsweetened cocoa powder, stirring until smooth. Stir in honey to sweeten the glaze. Pour the warm cocoa glaze over the top of the cake, allowing it to flow down the sides. Place the cake in the refrigerator for about 1 hour to allow the glaze to set properly.

After chilling, remove the Sokolatina from the fridge. Use a sharp knife to cut the cake into squares. Consider decorating each piece with additional whipped cream peaks or chocolate shavings. Serve chilled.