Tres Leches means three milks, which can make for an awfully heavy and dense cake when you think about it. But the magic elves in the Cook's kitchen made this work in big ways without falling to a soggy mess.
Put the condensed milk into a microwave safe bowl (use a big one). Cover tightly (no joke) with plastic wrap and microwave on low power for 9-15 minutes, stirring and replacing the plastic wrap every 3 minutes. Seriously, replace the plastic wrap every 3 minutes. Watch the microwave like a hawk. This goes from awesome to sticky hot mess all over your microwave in seconds. Not that I would know.
When slightly darkened and thickened, remove plastic wrap and slowly whisk in the evaporated milk, cream, and vanilla. Cover and set aside.
For the cake:
preheat the oven to 325 degrees and grease a 9x13 baking dish.
Whisk the flour, baking powder, salt and cinnamon in a bowl and set aside.
Melt the butter with milk in a small saucepan over low heat. Remove from heat to cool.
Using a stand mixer fitted with the whisk attachment, whip the eggs on medium speed until foamy, about 30 seconds.
Slowly add the sugar, and continue to whip until incorporated, 5 to 10 seconds. Increase the speed on that bad boy to medium-high and whip those eggs until they are thick and glossy, 5 to 7 minutes longer.
Reduce the speed and pour in the milk and butter mixture and vanilla.
Add the flour mixture, in three additions, scraping down the sides as needed with a rubber spatula.
Spread cake batter into prepared baking dish and bake for 30-35 minutes until toothpick inserted in the center comes out clean.
Cool on a wire rack for 10 minutes.
Using a wooden skewer (bigger than a toothpick), pole holes all over the top of the cake at 1/2" intervals.
Slowly pour the milk mixture over the cake until incorporated I poured a little at a time in different areas and let it be absorbed before continuing on.
Let sit at room temperature for 15 minutes before placing in the fridge for 3 to 24 hours, uncovered.
Make the whipped topping:
Remove the cake from the fridge 30 minutes right before serving.
Using a stand mixer (or a hand held mixer with the whisk attachment) whip the cream, corn syrup, and vanilla on medium low speed until foamy, about 1 minute.
Increase the speed gradually to avoid splatter and whip until soft peaks for about 2-3 minutes. Spread over cake, slice and serve.