Heat oven to 325 degrees F. Line the bottom of a 9x13 baking pan with parchment paper then spray the entire inside of the pan with nonstick baking spray with FLOUR; set aside.
In a small saucepan, melt butter with milk over low heat, stirring occasionally until butter is completely melted; refrigerate to slightly cool.
In a medium bowl, whisk together flour, baking powder, salt, and cinnamon; set aside.
In the bowl of a stand mixer fitted with the whisk attachment, whip eggs on medium speed for 30 seconds, then slowly whip in sugar until incorporated. Increase speed to medium-high and whip for 5-7 minutes until the mixture is very thick and glossy.
Reduce speed to low and slowly add melted butter mixture and vanilla. With the mixer still on low, add flour mixture one third at a time, scraping down bowl as needed, then mix on medium speed just until fully incorporated, about 30 seconds; don’t overmix.
Transfer batter to prepared pan and spread into an even layer.
Bake at 325 degrees F until toothpick inserted into center comes out clean, 45 to 55 minutes. Transfer cake to wire rack and let cool 60 minutes.
Meanwhile, make the milk mixture: Pour condensed milk into large microwave-safe bowl or large glass measuring cup and cover tightly with plastic wrap. Microwave on low power (10%), for 15 minutes, stirring and replacing plastic every 5 minutes. (Make sure you are microwaving on low after every time you stir otherwise the mixture will explode!) Remove bowl from microwave and whisk in evaporated milk, heavy cream, and vanilla. Refrigerate to cool.
After 60 minutes (cake will still be slightly warm), use a serving fork (or regular fork if you don’t have one), poke holes all over the top of cake (rows about ½-inch apart, see photos).
Slowly and evenly pour milk mixture over the cake. Let cake sit at room temperature for 15 minutes, then refrigerate UNCOVERED for at least 6 hours, best if overnight.