Preheat the oven to 350 degrees F. Set out a 9 inch pie pan. Roll the pie crust out into a 12 inch circle. Then fold the crust over the rolling pin and gently move the crust to the pie pan.
Use your fingers to crimp the edges of the pie crust around the top of the pan. Press the crust down against the top of the pan a little. Then place the pie crust in the freezer until ready to use. (This helps it to not slump while baking.)
Meanwhile, place the butter and sugar in the bowl of an electric stand mixer. Cream the butter and sugar together on high speed, until light and fluffy. Usually 3-5 minutes. Scrape the bowl with a rubber spatula.
Turn the mixer on low and add in the eggs and vanilla. Scrape the bowl again. Then turn the mixture on low and add in the flour, baking powder, and salt. Once combined, turn off the mixer. Do not overbeat.
Scoop the cake batter into the pie crust. Then bake on the lowest rack for 20-25 minutes, until the center is puffed and set.
For the Tres Leches: In a small bowl combine the sweetened condensed milk, evaporated milk, heavy cream, and cinnamon. Whisk well to break up the clumps of cinnamon.
Once the pie comes out of the oven, allow it to cool for 5 minutes. Then use a fork to prick holes all over the surface of the cake. This will allow the tres leches to absorb easily into the cake.
Slowly pour the tres leches over the cake. Take a break when needed to let the tres leches absorb. Then continue until all the tres leches is incorporated. Try not to let any of the milk mixture run over the sides of the crust. Place in the refrigerator.
For the Whipped Cream: Place the heavy cream and sugar in a clean mixing bowl. Beat on high until firm peaks form. Scrape the bowl and whip again if needed.
Spoon the whipped cream over the top of the pie. Spread into an even dome. Sprinkle the top with cinnamon for decoration. Then place back into the refrigerator until ready to serve.
As with traditional tres leches cake, this pie is best when eaten on the first or second day.