Bake the cake: Combine the flour, baking powder, baking soda and salt in a bowl. In a separate bowl, combine the buttermilk, strawberry jam and vanilla extract.
In a large bowl, whip the butter and sugar together with an electric mixer until pale and fluffy. Add the eggs and egg whites one at a time, mixing after each addition. Add the flour mixture to the egg mixture in 3 addition, alternating with the buttermilk mixture. Add the pink food coloring and mix until just combined.
Pour the batter into a greased, square 9' cake pan and bake at 350F for 45 min - 1 hour, or until fully cooked. Cool completely.
Make the buttercream: Cream the butter with an electric mixer until pale and fluffy. Add the vanilla extract and mix to combine. Add the confectioner's sugar one cup at a time, then beat for 3 minutes until fluffy. Add some pink food coloring and mix to combine.
To assemble: Slice the cake into 2 circles, then slice each circle in half to create a total of 4 layers. Spread some buttercream between each layer, then coat the entire surface of the cake in a thin layer of buttercream. Place in the fridge for 15 minutes for the buttercream to stiffen. Coat the cake in a thicker layer of buttercream. Stick pocky around the outside of the cake and tie a ribbon around the cake to secure them. Top the cake with fresh strawberries and enjoy!