Pre-heat your oven to 180°C (356°F) for a fan oven, 200°C (392°F) conventional, gas 6. Make sure all your butter is soft enough to work with, but not molten.
To make the Victoria sponge cake batter
Cream* together the butter and sugar. Add the flour, milk and baking powder and mix well. Add one egg at a time, beating well to mix it in. You should end up with a smooth, creamy batter that is not runny.
Butter the sides and bottom of your baking tins and pour half of the mixture into each one. You should have a depth of 2-3 cm (one inch, more or less). Place the tins into the oven and bake for 25 minutes. When the sponges are done, they should spring back when you gently press the top.
Remove from the oven and get them out of the tins as quickly as you can - you may need to wait about 5 minutes until the tins are cool enough to handle. Place them on a wire rack to cool completely.
To make the Victoria sponge cake filling
Cream together the butter and sugar. Open the jar of jam. See, I told you cake-making wasn't hard!
Assemble the Victoria sponge cake sandwich
When your sponges are cool, decide which will be the top and which the bottom - basically you want the one with the best-looking top to be the top. With a sharp knife, carefully slice the top off the bottom sponge to leave a level suface for the filling. Spread a generous layer of jam onto the sponge, and then a thick layer of buttercream. Place the top sponge on, and sprinkle with sifted caster- or icing sugar. Congratulations, you just baked a Victoria sponge sandwich cake!
*About creaming
If you haven't made a cake before, you need to know about creaming. It's basically mixing butter and sugar and working air into it. A stand mixer with a paddle attachment will do this in a minute, but if you don't have one, a wooden spoon, a hand whisk and some elbow grease will do it. You should end up with a mixture that is light, soft and creamy.