Generously butter a couple 8-inch round cake pans. Line the base of each pan with a circle of baking parchment cut to fit. Preheat the oven to 350°F (176°C).
Beat the butter and sugar together until fluffy and almost white in color. This is easiest with an electric beater but you can instead use a wooden spoon.
Lightly whisk the eggs together in a small bowl. Add to the butter mixture a little at a time, beating constantly. (Beating in the eggs a little at a time will mostly prevent the mixture from appearing curdled, although don’t worry if the mixture does appear so, because as soon as the flour is added it will bring it back together. The texture may be slightly denser, but the cake will be fine.)
Now sift in the flour directly onto the mixture in 3 or 4 additions, folding each in carefully so you don’t knock out all the air you have beaten in. (You want to get under the butter mixture, turning it over rather than stirring it, and going around the sides and through the middle as gently as possible until all the flour is mixed in.) Divvy the batter between the 2 cake pans and level the surface. Place on the middle rack of the oven and bake for 25 to 30 minutes, until well risen and golden brown. To check if the cakes are done, press one gently in the middle with your finger—it should spring back up. If you’re still not sure, insert a skewer in the center—if it comes out clean, the cake is cooked through. Remove the cakes from the oven and leave in the pans for 10 minutes. Then run a knife around the edge of each one to loosen it if necessary. Turn out the cakes onto a wire rack. Let cool completely. (You can store the sponge cake in a resealable container for up to 1 week.)
Peel off the baking parchment and place one of the cakes on a serving plate. Spread the raspberry jam over the top of that cake. Plonk the other cake on top of the jam, then sift the confectioners' sugar over the top through a fine sieve.