#### Make the Chocolate Sponge Cake
**Preheat the oven**
Preheat your oven to 350°F (175°C). Line a 10x15-inch jelly roll pan with parchment paper, leaving some overhang for easy removal.
**Whip egg yolks and sugar**
In a mixing bowl, beat the egg yolks with 1/4 cup of granulated sugar until pale and thick, about 3–4 minutes. Add vanilla extract and mix to combine.
**Beat egg whites**
In a separate clean bowl, beat the egg whites until soft peaks form. Gradually add the remaining 1/4 cup sugar, beating until stiff peaks form.
**Fold ingredients**
Gently fold the whipped egg whites into the egg yolk mixture in batches. Sift the cocoa powder, flour, and salt over the mixture, then gently fold until just combined.
**Bake**
Spread the batter evenly into the prepared pan. Bake for 10–12 minutes, or until the cake springs back when lightly touched.
**Cool and roll**
Turn the warm cake onto a clean kitchen towel dusted with cocoa powder. Carefully peel off the parchment paper. Starting at the short end, roll the cake with the towel into a log shape. Let cool completely.
#### Make the Chocolate Buttercream Filling
**Cream butter and sugar**
In a mixing bowl, beat the softened butter until smooth. Gradually add powdered sugar and cocoa powder.
**Add vanilla and cream**
Mix in heavy cream and vanilla extract until smooth and fluffy.
#### Assemble the Yule Log
**Unroll the cake**
Carefully unroll the cooled cake. Spread the buttercream evenly over the surface, leaving a small border around the edges.
**Roll the cake**
Re-roll the cake tightly (without the towel) and place seam-side down on a serving platter.