In a medium bowl, combine the flour, brown sugar, salt and cinnamon by stirring with a spoon. Add the butter chunks, and use your hands to “smush” the butter into the flour mixture until it resembles coarse sand. Place the crumb topping in the fridge until ready to use.
For the cake:
In a large bowl, combine the butter and sugar with a hand mixer on medium-high speed until light and fluffy, about 2 minutes. Add the egg, buttermilk, and vanilla and mix until well incorporated, about a minute. Add the flour mixture and mix on low speed until just combined, scraping the sides of the bowl down as needed. Be careful not to overmix.
Scrape the batter into the prepared baking pan and smooth the top into an even layer. Place the peach slices in circles on the top of the batter. Sprinkle the reserved crumb mixture on top of the peaches.
Place the cake in the oven and bake for 50-60 minutes or until a toothpick inserted into part of the cake in the center comes out clean (be careful not to place the toothpick inside of a peach). Remove the cake from oven and let cool on a wire rack for 15-20 minutes before slicing. If using a cheesecake pan, let the cake cool for 20 minutes, run a knife along the outer rim, then carefully remove the ring. Serve the cake warm or at room temperature.
Leftovers will keep in the fridge for up to 5 days. Reheat slices of cake in a 350℉ oven for 8 minutes.
*Nutrition information is calculated based on 1/10 of the cake.