Preheat the overn to 180C, fan 160C. Liberally grease a deep 20cm diameter fixed-base cake with butter and line the base. For the topping, put the caster sugar in a medium heavy-based saucepan and heat gently until it has melted; don't stir, but tilt the pan occasionally. Continue to cook until it turns a deep golden brown. Add the butter and whisk until it melts, then add the bourbon, taking care as it will splash up. The sugar will seize but just continue whisking it over a low heat until it melts into the caramel. Continue to bubble for 2-3 minutes, or until you have a syrupy consistency. Set aside while you make the cake.
For the cake, beat the butter and sugar together until light and fluffy. Add the eggs, one at a time, beating well between each addition. Combine the plain flour, ground almonds, baking powder and bicarb, plus a pinch of salt, then add to the main mixture, folding gently until all the ingredients are combined. Lastly, fold in the buttermilk and vanilla.
Drizzle two-thirds of the caramel over the base of the lined tin then add the peach slices in a circle, filling the centre with a couple of slices. Spoon the cake batter over and level the top. Bake in the oven for 55-60 minutes, or until a cake skewer comes out clean.
Remove from the oven and let the cake stand for 10 minutes before inverting onto a plate. Gently reheat the remaining caramel; when warm, brush over the cake to glaze. Serve warm, with ice cream.