Put peaches in a large bowl. Add brown sugar, cinnamon, ¼ cup granulated sugar and toss to combine. Let sit at least 20 minutes while you prepare the batter.
Preheat oven to 350⁰F.
Slice butter into pieces and add to a 9x13 inch baking dish. Place the pan in the oven while it preheats, to allow the butter to melt.
In a large bowl mix together the flour, ¾ cup sugar, baking powder, and salt. Stir in the milk and vanilla, just until combined.
Remove the pan with melted butter from the oven and pour the batter into the baking dish pushing it into the corners, but do not mix with the butter. Just let the butter swirl around the batter.
Top with the peaches. Sprinkle a little extra cinnamon on top. Bake at 350 degrees for 40 – 45 minutes until golden brown and bubbling around the edges.