Preheat the oven to 180°C/160°C fan/350°F/gas mark 4. Line a 20 cm / 8 inch square cake tin with baking parchment, making sure that you leave some overhanging so that you can easily lift the baked brownies out of the tin.
Start by making the shortbread. Whisk together the softened vegan butter and the caster sugar with an electric mixer until it is light and fluffy, then whisk in the vanilla extract and a good pinch of salt.
Sift together the plain flour and cornflour (cornstarch) then add them to the butter mixture. Stir with a spatula until it forms a dough, it may seem like it is too much flour but it will combine eventually.
Tip the dough into the prepared cake tin and use your hands to pack it down as firmly and as evenly as possible.
Bake the shortbread for about 20 mins until it is golden brown. If it has puffed up at all press it back down gently.
Once the shortbread is baked prepare the brownies. Sift together the flour, cocoa powder, espresso powder and salt.
Place the butter, sugar and milk in a pan over a low heat. Stir until the butter is melted and the sugar is fully dissolved. It should be hot but not yet boiling.
Remove the pan from the heat and add the vanilla extract and chopped chocolate. Whisk until the chocolate is fully melted.
Add the sifted dry ingredients and stir until fully combined.
Spread the mincemeat evenly over the shortbread base.
Pour the brownie batter over the mincemeat and spread it level.
Bake for 35-45 minutes until set and a skewer inserted into the centre comes out gunky with moist crumbs but not too much wet batter.
Allow the brownies to cool completely in the tin before lifting them out and slicing into squares.