Slit the vanilla beans lengthwise and scrape out the seeds.
Cream together the vanilla extract, softened butter, ½ cup sugar, and salt using a whisk or mixer. Beat in the 6 eggs one at a time. Mix the flour with the baking powder and stir it into the batter along with the milk.
Pour the batter onto the prepared baking sheet and spread evenly. Scatter the blueberries over the top. Bake for 20 minutes.
Beat the 4 egg whites until stiff peaks form, gradually adding ¾ cup sugar and vanilla extract.
Remove the cake from the oven and spread the meringue over it. Bake for another 15 minutes at the same temperature.
Remove the cake from oven and let it cool completely. Cut the cake into 24 pieces and serve.