Preheat oven to 350°F (177°C). Spray or lightly butter a quarter sheet pan. Set aside.
Make the cake: Whisk the flour, baking powder, and salt together in a bowl. Set aside.
With a handheld or stand mixer fitted with a paddle attachment, beat the butter with the sugar on medium speed in a large bowl until creamy, about 4 minutes. With the mixer running on low speed, add the eggs one at a time. Beat on medium-high speed until combined.
With the mixer running on low speed, add the flour mixture and 1/2 cup of liquid from the canned peaches.Don't overmix.
Pour batter evenly into the quarter sheet pan. Bake for around 20 minutes or until a toothpick comes out clean. Allow cake to cool completely.
Make the Vanilla Pudding: Pour the milk into a small pot, add 1/4 cup granulated sugar, 1 egg yolk, vanilla extract and 2 tbsp cornstarch. Whisk everything together until smooth then turn on the heat and whisk constantly until the mixture starts to boil. Let it boil for 30 seconds. The cornstarch will thicken the mixture and makes the pudding creamy. Turn off the heat and let the pudding cool down. To prevent that a skin forms on the pudding continue stirring occasionally or press plastic wrap onto the surface of the pudding.
In the meantime drain the peaches and dice half of them into small pieces. These will go into the Vanilla Sour Cream Mixture. Puree the other half in a food processor. Set aside.
When the pudding is at room temperature add the sour cream and the diced peaches. Whisk until combined.
Assemble the dessert in glasses: Use a cookie cutter or a glass to cut out circles with the same diameter like the glasses you want to use for this dessert. Put the cake layer into all your glasses. Mine had a diameter of 3 inches and I got 12 circles out of a quarter sheet pan. Divide the Vanilla Sour Cream Mixture between the glasses, leave a little bit of room for the peach puree. Use a spatula or a spoon to smooth the Vanilla Mixture then add the puree on top.
Cover the glasses with lids or plastic wrap and put them in the fridge! You can make this dessert one day in advance, just keep the glasses in the fridge until you are ready to serve them.