To make the base, melt the butter, then sizzle until it turns a nutty brown if you want some extra flavour. Blitz the biscuits into crumbs using a food processor, then add the melted butter and blend to mix. Press the crumbs into a lined 20cm square tin, then chill until set.
Make the filling by beating the butter and sugar together until light and fluffy. Add the eggs one at a time, tip in the flour and almonds and continue beating, then spoon into a piping bag.
Heat oven to 180C/160C fan/gas 4. Remove the tin from the fridge, then spread over the jam, leaving a gap around the edge. Pipe the filling mixture on top, making sure you start with the edge around the jam. Press in the blackberries, sprinkle over the caster sugar and bake for 45-50 mins. The frangipane should be cooked all the way through with a sugary crust on the top. Remove from the oven and leave to cool. Can be made the day before and kept, wrapped, at room temperature. Bring to the picnic in the tin, then cut into squares and serve with a large spoonful of clotted cream.