Prepare sponge: Preheat oven to 160°C fan (180°C static). Cream butter and sugar until pale, beat in eggs one at a time. Fold in flour, baking powder, salt, lemon zest, and milk .
Bake: Divide between two tins, bake 35–40 minutes. Cool thoroughly.
Make frosting: Beat butter, gradually add icing sugar, then tint with blue gel until just pastel.
Assemble: Level cakes, fill with buttercream, crumb‑coat, chill, then apply final coat; smooth with a spatula dipped in warm water .
Speckle effect: Mix cocoa with vanilla (or water), dip a stiff paintbrush or spoon, then flick over the cooled cake for speckles.
Top: Press mini eggs on top in a cluster or nest; chill briefly to set.