In a bowl, mix together the cake batter and pistachios. Pour onto a baking sheet lined with a silicone baking mat. Transfer to the oven and bake until golden brown and crispy, about 10 minutes. Let cool completely, then transfer to a food processor and pulse to a coarse crumble.
Peach Upside-Down Cake:
Preheat the oven to 350 degrees F.
Place 1 tablespoon of the butter in 8 ramekins, and rub all around the inside. Divide 1/4 cup of the sugar between the ramekins and turn so the insides are coated with sugar. Divide the peaches between the ramekins.
In a large bowl, combine the remaining 1/4 cup sugar and the flour, baking powder, baking soda and salt; stir to incorporate. In a separate bowl, combine the buttermilk, vanilla, eggs and lemon zest. Melt the remaining stick butter in a small saucepan and cook until brown; pour into the wet ingredients and mix together with a whisk. Add the wet mixture to the dry, and stir with a spatula until combined.
Pour the batter over the peaches, filling to just below the top of the ramekins. Set the ramekins on a small baking sheet and transfer to the oven. Bake for 25 minutes. Let cool for 7 to 10 minutes.
Run a paring knife or small offset spatula around the edge of each ramekin, and then invert the cakes onto serving plates.