Preheat oven to 200 degrees C. Line a large rectangular pan (about 30 x 40 cm or 12 x 17 inch) with a sheet of baking paper.
In a bowl combine flour and cornstarch and set aside.
Divide 3 of the eggs by placing the yolks in one large mixing bowl and the whites in another. Add to the yolks the last egg and about 2/3 of the sugar. Beat with the mixer at high speed for 5 minutes until the volume increases significantly and the mixture becomes thick. Add vanilla and beat until incorporated.
Gradually sprinkle the flour mixture in the egg yolks, folding it gently with a rubber spatula until absorbed.
Beat the egg whites until foamy. Add cream of tartar/ lemon juice and beat until soft peaks form. Add the remaining sugar and beat until hard peaks form.
Carefully fold the egg whites into the batter.
Pour the batter into the prepared pan. Distribute it evenly and smooth it out with a spatula.
Bake for 7-10 minutes or until a toothpick inserted in the centre comes out clean and the cake is firm.
Remove the cake from the pan together with the paper and place it on top of a clean tea towel. Roll it up together with the towel and with the paper and let cool for 30 minutes.
Unroll the cake, remove the baking paper and roll it up again together with the tea towel. Let it cool until room temperature before assembling.
Apricot Buttercream Frosting:
In a food processor puree apricots until smooth.
In a large mixing bowl beat butter until fluffy. Little by little add sugar. Continue beating until the sugar is absorbed and the mixture becomes creamy and fluffy. Little by little add the apricot puree. Beat until it is absorbed. The frosting should remain fluffy.
When the cake is completely cool, unroll it and spread evenly about 2/3 of the buttercream frosting on top. Roll up the cake tightly. Frost with the remaining buttercream frosting on top and on the sides. Cover with the chocolate glaze and decorate with chopped apricots.
In a heatproof bowl over gently simmering water melt the chocolate together with the milk.