Preheat the oven to 350°F (177°C). Line a baking sheet (16.5 x 11.38 x 1 inches) with parchment paper or a silicone mat.

In a mixing bowl with a whisk attachment, beat the egg whites on medium speed until soft peaks form, about 3-5 minutes.

In a small bowl, combine milk and melted butter.

Sift the cake mix into the meringue and carefully fold until smooth using a silicone spatula. Pour in the milk and butter mixture and gently fold until smooth.

Pour the batter into the prepared baking sheet and spread it evenly.

Bake for 12-15 minutes, or until a toothpick inserted comes out clean and the cake is lightly golden.

While the cake is baking, prepare a lint-free kitchen towel on a flat surface and dust generously with powdered sugar.

Invert the cake onto the prepared kitchen towel and tightly roll it from the short side while still warm. Cool completely on a wire rack, about 1 hour.

Microwave ¼ cup of heavy whipping cream until hot, about 30 seconds. Pour over chopped chocolate and let stand for a minute, then stir until melted. Cool completely.

In a mixing bowl with a whisk attachment, beat the remaining 1 cup of heavy whipping cream until hard peaks form. Fold in the cooled white chocolate mixture until well combined.

Once the cake is completely cooled, carefully unroll it and generously spread white chocolate mousse all over, reserving about ½ cup of filling.

Tightly roll the cake and trim off the ends. Frost with the reserved mousse filling and sprinkle crushed M&Ms on top if desired.

Refrigerate until ready to serve. The cake is best served the same day.