Prepare the syrup according to the recipe. In a small container, mix the milk and egg yolk until smooth. Then pour the resulting liquid into a sieve and add sugar.
Place the container with this mixture on the fire. Cook for 7-8 minutes, stirring constantly. Once the mixture boils, cook for another 1-2 minutes. Pour the syrup into another container and cover with cling film. The syrup, prepared at home, should resemble condensed milk in consistency.
Make the sponge cake cream. In a large bowl, beat the softened butter with a mixer until fluffy and light in color. Gradually add the cooled syrup. The finished sponge cake cream should be white and thick, but not dense.
Flavor the cream for the sponge cakes. Add cognac to the finished mass and mix thoroughly.