How to Make It
In a stand mixer fitted with the paddle, beat all of the ingredients together at medium speed until crumbs form. Transfer the crumbs to a bowl and press into clumps. Refrigerate until chilled, about 15 minutes.
Set up a gas grill for indirect grilling, then heat to 300°. In a medium bowl, toss the peaches with the blueberries, sugar, lemon juice and cornstarch. In a 12-inch cast-iron skillet, melt the butter over low heat. Remove from the heat. Scrape the fruit and any juices into the skillet.
In a medium bowl, whisk both flours with the graham cracker crumbs, baking powder and salt. In a stand mixer fitted with the paddle, beat the butter with both sugars and the honey at medium speed until fluffy. Beat in the eggs one at a time, then beat in the buttermilk, oil and vanilla. Scrape down the side of the bowl and beat in the dry ingredients until just smooth. Spread the batter in the skillet in an even layer. Scatter the streusel evenly on top.
Set the skillet on the grill over indirect heat. Close the grill and bake for about 1 hour and 30 minutes, rotating the skillet every 20 minutes, until a toothpick inserted in the cake comes out clean; keep an eye on the heat to maintain the grill temperature. Let the cake cool for 1 hour. Cut into wedges and serve with vanilla ice cream.