Line a loose-based square brownie tin (20cm/8") with greaseproof non-stick paper and set to one side. Preheat your oven to 170°C/325°F/gas 3. If using a fan assisted oven set it to 150°C.
Stir together all of the ingredients for the shortbread base into a large mixing bowl. Add in the softened butter/shortening and rub together with your fingers until the mixture resembles fine breadcrumbs.
Knead the mixture together until it forms a dough, then press into the lined tin in a nice even layer. Place your shortbread into the freezer for 10 minutes until firm.
Remove the shortbread from the freezer and prick with a fork. Place onto the middle shelf of the oven and bake for 20-25 minutes until a light golden brown colour. Remove and leave to cool completely in the tin.
Place all of the ingredients for the condensed milk into a medium heavy-bottomed pan. Place on a low heat and bring to a simmer. Gently simmer for 30-40 minutes until the coconut milk has darkened and thickened, stirring occasionally. The mixture should have reduced by roughly half. (*If using shop-bought coconut condensed milk skip this step now!)
Add your remaining caramel ingredients, except for the matcha powder, into the condensed milk and whisk until combined. Place on a low heat until the sugar has dissolved then up to medium heat and bring to the boil. Reduce the heat and simmer, stirring continuously, for about 15-20 minutes or until the mixture has thickened considerably.
Whisk the matcha powder into a little hot water until smooth then transfer to the caramel and whisk until combined. Bring to a boil for 1 minute then remove from the heat. Pour over the shortbread immediately then transfer the tin to the freezer for 30 minutes until set.
Once your caramel has cooled melt the chocolate in a double boiler. Leave to cool for 5 minutes before pouring over the caramel layer. Leave to cool for a further 5 minutes before topping with sprinkles, then leave to cool completely before freezing for 15 minutes until firm.
Using a hot, wet knife score lines into the set chocolate until it breaks through to the caramel. Wipe your knife using a clean damp cloth and carefully slice through to the base. be sure to wipe your knife clean after every cut. Make sure you slice whilst still in the tin then remove your slices carefully. Store in an airtight container for up to 5 days within the refrigerator or freeze for up to 1 month.
Recipe by Greens of the Stone Age at https://greensofthestoneage.com/spooky-matcha-millionaires-shortbread-paleo-vegan/