Preheat the oven to 325 degrees F. Grease an 8 x 8 brownie pan with butter.
Melt the butter and set aside.
In a small pot, combine the milk, condensed milk and granulated sugar until lukewarm.
Using a stand mixer, whip the egg whites and vinegar until stiff peaks form.
In a larger bowl, whisk the egg yolks and the confectioners sugar together until it turns a pale yellow color, about 3 minutes.
Add the melted butter and mix. Next add the lime juice and combine.
Add the milk mixture gradually whisking well each time.
Add the flour and mix well making sure there are no lumps.
Finally fold in the beaten egg whites in 3 stages making sure to get rid of most of the lumps but still try to keep it on the lighter side.
Once all of the egg whites have been mixed in, pour the batter into the prepared pan.until about an inch from the top.
Place in the upper part of the oven and bake for 50-55 minutes. The top should be still slightly wobbly when done but you'll see small cracks appear around the edges of the cake. It will continue to firm up as it cools down.
Dust heavily with extra powdered sugar before slicing and serving.