Preheat oven to 350°F. Line a 9x13 inch baking pan with parchment paper, letting the excess hang off of the sides of the pan.
In a large bowl, whisk together the brown sugar, baking powder and salt. Gradually add the melted butter and stir until just combined.
Stir in ¼ cup of bourbon and vanilla. Add the eggs one a time, whisking well after each addition. Stir in the flour until just combined.
Fold in the peach preserves and then fold in ¾ cup of the diced peaches. Spread batter into the prepared pan and top with remaining ¼ cup of peaches.
Bake until a wooden toothpick inserted into the center comes out with just a few moist crumbs, about 35 to 40 minutes. After 30 minutes of baking, you can cover lightly with a piece of aluminum foil to prevent over browning.
Let cool completely on wire rack.