With the rack in the middle position, preheat the oven to 350°F (180°C). Butter two 8-inch (20 cm) springform pans and line the bottoms with parchment paper. Set aside.
In a bowl, combine the flour, baking powder and salt. Set aside.
In another bowl, beat the eggs, sugar and vanilla with an electric mixer for about 10 minutes until the mixture triples in volume and forms a ribbon as it falls from the beaters. Drizzle in the oil while beating.
At low speed, stir in the dry ingredients alternately with the milk until smooth. Spoon the batter into the prepared pans.
Bake for 55 minutes or until a toothpick inserted into the centre of the cake comes out clean. Let cool completely
on a wire rack. Unmould.
Peanut Butter Meltaways
Line the bottom of two 8-inch (20 cm) springform pans with parchment paper.
In a bowl placed over a pot of simmering water or in a microwave, melt the chocolate. Remove the bowl from the heat and stir in the peanut butter. Pour the mixture into the prepared pans. Refrigerate for 30 minutes or until firm. Run the blade of a knife between the peanut butter meltaways and the pans and unmould. Keep on parchment paper in the refrigerator until ready to assemble.
Cut and discard the rounded tops of each cake. Cut each cake horizontally to obtain 4 slices total.
On a serving dish, place 1 slice of cake. Spread with 3/4 cup of the buttercream, sprinkle with 1/4 cup of the peanuts and top with 1 disc of peanut butter meltaway. Layer another cake slice and spread with 3/4 cup of the buttercream. Sprinkle with 1/4 cup of the peanuts, 1/2 cup of the caramel popcorn and drizzle with half of the caramel sauce. Repeat the sequence, finishing with the remaining popcorn and caramel sauce (see note).
The cake will keep for 2 days at room temperature.
The cake, peanut butter meltaways and caramel buttercream can be made a day in advance. However, it is best to top the cake with popcorn and caramel sauce just before serving.