Preheat the oven to 350°F (175°C). Line a 10×15-inch jelly roll pan with parchment paper and lightly grease it.

Make the sponge cake: In a large bowl, beat eggs and sugar on high for 5–6 minutes, until light and fluffy. Mix in vanilla, oil, and milk. In a separate bowl, sift together flour, baking powder, and salt. Gently fold dry ingredients into the wet. Add blue gel food coloring to reach your desired robin’s egg shade.

Bake: Pour the batter into the prepared pan and spread evenly. Bake for 10–12 minutes, or until the cake springs back when lightly touched.

Roll the cake: Immediately after baking, turn the cake out onto a clean kitchen towel dusted with powdered sugar. Peel off the parchment, and roll the cake gently (with the towel inside) from the short end. Let cool completely.

Make the filling: Whip the cream, powdered sugar, and vanilla until stiff peaks form (or prepare buttercream). Unroll the cooled cake and spread the filling evenly over it. Reroll the cake without the towel.

Decorate: For the speckled effect, mix cocoa powder with a little water or vodka to create a thin paste. Use a stiff brush or clean toothbrush to flick the mixture onto the cake for a speckled look. Chill to set.