01 - Preheat oven to 325°F. Grease and flour a 12-cup bundt pan thoroughly.
02 - In a small saucepan, combine 1 cup diced peaches, light brown sugar, ground cinnamon, and cornstarch. Cook over medium heat for 5 minutes, stirring frequently, until the mixture thickens. Remove from heat and allow to cool.
03 - In a large bowl, beat unsalted butter and granulated sugar together with a mixer on medium speed for 3–4 minutes until light and fluffy.
04 - Add eggs one at a time, beating well after each addition.
05 - Mix in sour cream and vanilla extract until evenly combined.
06 - In a separate bowl, whisk together all-purpose flour, baking powder, and salt.
07 - Gradually mix dry ingredients into the creamed butter mixture until just combined.
08 - Gently fold in 2 cups diced peaches and ground cinnamon until evenly distributed.
09 - Spoon half of the batter into the prepared bundt pan. Swirl half of the cooled peach filling over the batter. Add the remaining batter, then swirl in the rest of the peach filling.
10 - Smooth the top and tap the pan gently to remove air bubbles. Bake for 70–80 minutes, or until a toothpick inserted into the center comes out clean. If the cake browns too quickly, loosely cover with foil after 50 minutes.
11 - Remove from the oven and cool in the pan for 20 minutes. Invert the cake onto a wire rack and allow to cool completely.
12 - Puree 1/2 cup diced peaches, then whisk with powdered sugar and enough milk to create a thick yet pourable glaze. Drizzle over the cooled cake before serving.