Preheat the oven to 375 degrees Fahrenheit (180 ºC). Adjust the oven rack to the lower middle position.
In a stand mixer beat the eggs, cream of tartar, sugar, and salt for about 3-5 minutes. It should triple its volume and soft peaks form.
In a medium bowl sift and mix the flour with the baking powder.
Turn off the mixer and then add the flour mixture a few spoonfuls at a time and with a silicon spatula slowly fold in with soft movements until is well combined.
Pour batter into a prepared 8×8 inch pan and bake for about 15 minutes and make sure it is done by inserting a toothpick.
Let it cool on a cooling rack while you are making the cream.
In a medium saucepan, pour the condensed milk and egg yolk and mix.
In the condensed milk can mix the milk and the cornstarch well.
Pour it into the condensed milk and mix until well combined.
Bring to a simmer over low heat while constantly mixing with (a spoon or whisk) to avoid scorching until it’s thickened. It is supposed to be pretty thick.
Then put it away from heat until it cools completely.
Once the cake and the cream have cooled, cut the cake into small dices.
In a large clear bowl make a layer of cake pieces. Then add a layer of cream and then cover the cream with the mixed berries. Ad as much as you like calculating well the layers you are planning on making.
Repeat cake, cream and finish with whole strawberries and raspberries to decorate.
Leave it on the fridge for a couple of hours covered with plastic wrap.
Serve and enjoy!!!