Preheat oven to 180 °C. Line the bottom of two 20cm cake tins with baking paper. Grease the sides with butter.
Add first measure of butter and caster sugar to a large bowl. Using hand-held electric beaters, beat butter and sugar together on high speed for about 4 minutes, until pale and fluffy. Add eggs one at a time, beating well after each addition, until incorporated. Add first measure of vanilla extract and beat again, until combined.
Add sponge mix to egg mixture and gently fold in, until just combined and no dry spots remain (to keep your cake from sinking, fold in flour with a metal spoon and do not over mix). Add milk and mix, until just combined. Divide sponge mixture between the two prepared cake tins and smooth out into an even layer with the back of a spoon.
Bake in oven for about 20 minutes, until risen and golden. (If your cake tins don't fit on the same rack, simply put one on the top rack and one on the bottom. Refrain from opening the oven door until cooked.) Remove from oven and leave to cool in tins for about 5 minutes. Run a knife around the outside of the cakes in the tin, then turn out onto a wire rack to cool completely.
Use electric beaters to whip cream with icing sugar and vanilla, until soft peaks form, about 3 minutes.
Once sponges have cooled completely, spread the base with raspberry chia compote and top with cream. Sandwich together with second sponge.
Cut Victoria sponge cut into slices and serve with a nice cup of tea.