Preheat oven to 350℉. Place ¼ cup salted butter in a 10" round non-stick cake pan. Place pan in oven for a few minutes until the butter melts. Remove pan from oven and mix in the 1 cup brown sugar. Using an off-set spatula spread evenly on the bottom of the pan. Arrange the peach slices in a spiral pattern evenly over the brown sugar mixture. Set aside.
Cake:
Cream butter and sugar together on medium speed with electric hand mixer or stand mixer until pale and creamy. Add in the eggs one at a time until well incorporated. Mix in vanilla.
In a separate bowl whisk together the flour, baking powder and salt. Add alternately to the creamed mixture with the milk on low speed - do not over beat.
Spread batter evenly over the peaches being careful not to disturb the pattern.
Bake for 35-40 minutes or until toothpick inserted in centre comes out clean. Cool for 10 minutes on cooling rack and then invert on to serving plate.
Recipe by Chateau Elma at https://www.chateauelma.com/peach-upside-down-cake