Melt 50g butter with 50g sugar, add 500g blackberries and lemon juice, and cook on low heat until soft.
Blend the blackberry mixture until smooth, pass through a fine sieve, and set aside.
Bring cream and milk to a boil in a large saucepan.
Whisk egg yolks and caster sugar, then pour in the hot cream mixture while whisking continuously.
Cook the custard gently until it reaches 82°C, sieve into a bowl, and chill thoroughly.
Mix 400g of the blackberry purée with the chilled custard and sieve again.
Pour mixture into an espuma gun, charge with two chargers, shake well, and rest for 15 minutes.
Combine dry biscuit ingredients with 60g butter until crumbly, then mix in egg and syrup to form a paste.
Wrap biscuit dough and chill in the fridge for at least one hour.
Preheat your oven to 180°C and line two baking sheets with parchment.
Form the dough into small balls, place 5cm apart, and bake for 12 to 15 minutes.
Cool biscuits on a wire rack until completely firm.
Layer blackberries, purée, crumbled biscuits, and ice cream into six glasses.
Spray the blackberry fool from the gun over the top, dust with mace, and serve.