Preheat the oven to 350°F (175°C). In a large bowl, whisk together flour, sugar, baking powder, and salt. In another bowl, beat eggs and vanilla extract until fluffy. Gradually combine the wet and dry ingredients. Pour the batter into a lined 10x15 inch baking pan and bake for about 15-20 minutes until golden. Let cool, then roll into a log shape with parchment paper.

Preparation of Raspberry Chocolate Mousse

Prepare the Mousse

Melt the dark chocolate over a double boiler and let it cool slightly. In a separate bowl, whip the heavy cream until soft peaks form. Combine raspberry puree and sugar, then fold into the whipped cream. Dissolve gelatin in water and mix with the melted chocolate. Fold the chocolate mixture into the raspberry cream until well combined. Refrigerate until set.

Preparation of Butter Cream Frosting

Make the Butter Cream

In a bowl, beat the unsalted butter until creamy. Gradually add the powdered sugar and continue to beat until smooth. Mix in vanilla extract and heavy cream until the desired consistency is reached.

Assembly

Assemble the Yule Log

Unroll the sponge cake carefully. Spread a layer of raspberry chocolate mousse over the cake, then roll it back up tightly. Frost the outside with butter cream frosting. Decorate with fresh berries on top. Chill before serving.