Preheat oven to 180c/160c fan for a dark baking dish or 160c/140c for glass. Grease a 9x13 baking dish and set aside.
Mix the flour, baking powder, and salt together in a bowl. In a stand mixer or with a hand mixer, whisk the egg whites on low for a minute and increase for about 5 minutes until stiff peaks form. Carefully move the egg whites to a separate bowl with a spatula.
Add the egg yolks to the same mixing bowl that the whites were previously in along with the sugar. Whisk on medium for 2-3 minutes until the egg becomes pale. Lower the speed and add the milk and vanilla until combined.
Add the dry ingredients to the milk and yolk mixture and stir until combined. Carefully fold in the egg whites until there are no large lumps.
Pour the batter in the pan and smooth over the top with a spatula. Bake for about 20 minutes until the cake is springy and a toothpick can be inserted with few crumbs when it comes out.
Set the dish on a wire rack to cool completely. The cake may shrink slightly as it cools.
While your cake is cooling, mix together all of the ingredients for the milk and coffee mixture. When the cake is cooled, use a fork to poke holes throughout the cake. This will help the liquid to evenly soak through the cake and reach the middle. Pour the coffee and milk mixture over the cake as evenly as possible. There will be excess liquid but this will all be absorbed over time. Cover the cake and place it in the fridge for a minimum of 4 hours but I suggest doing this over night.
Before serving, whip the cream ingredients together in a stand mixer for 4-5 minutes until peaks form and you have a thick and spreadable mixture. Use a spatula to spread the mixture across the top of the cake. Garnish with cocoa powder and sliced strawberries before serving.