Preheat the oven to 350℉ (325℉ convection). Prepare baking pan by thoroughly greasing the inside bottom and sides of the pan with softened butterm then dusting with flour. Shake off the excess. Be sure to grease the pan really well to avoid the cake sticking.
In a small bowl, whisk together flour, baking powder, and salt. Set aside.
In another bowl, using an electric mixer fitted with a paddle attachment, cream together softened butter, salt, and ½ cup of sugar until light and fluffy.
In a separate bowl, using an electric mixer fitted with a whisk attachment, whisk together on medium-high speed the remaining ½ cup sugar and 4 eggs for 2 minutes until mixture is lighter in color and bubbly.
Add the egg mixture into the creamed butter and sugar mixture, add in vanilla extract then mix on low-medium speed until combined. It's okay if there are small lumps. Mix in dry ingredients until combined, then add the whole milk and mix on low speed until mixture turns into a uniform cake batter - avoid overmixing at this point.
Pour the cake batter into the prepared/greased pan and bake on the middle rack of the oven for around 22 minutes or until baked through and the top is lightly golden brown.
Pull the cake from the oven. Allow it to cool in the pan for at least 30 minutes. Using a fork, poke holes all over the top of the cake.
In a bowl whisk together the three milks - evaporated milk, sweetened condensed milk, and heavy cream for the milk soak. Pour milk mixture over top of the cake covering the entire surface. The milk mixture will look like a lot and puddle around the edges and top of the cake. Spread it around the best you can. Cover the pan with plastic wrap and place the cake in the refrigerator to chill for at least one hour.