Using the largest side of a box grater (or a mixer/food processor with a grater attachment), grate 1/2 cup of cold butter. Spread a thin layer over parchment or aluminum foil, and place in a freezer-safe airtight bag. Freeze overnight or at least for 3 hours.
Preheat oven to 350°F (177°C).
Spread chopped walnuts on a sheet pan and toast 6-8 minutes until fragrant.
Cool completely. Increase oven to 400°F (204°C). Line a baking sheet with a silicone baking mat
Whisk together the wet ingredients: butternut squash purée, cultured coconut, vanilla bean seeds, and walnut milk. Refrigerate 1 hour.
In a large bowl whisk together the dry ingredients: gluten-free multi-purpose flour, sugar, baking powder, (optional baking soda), salt, cardamom, and mandarin orange zest.
Cut In The Butter: Add in the frozen grated butter to the dry mixture. Use a pastry cutter to cut it in until you have a mix of pea-sized bits and smaller sandy pieces.
Stir in 1/2 cup of the toasted walnuts. Reserve 3 tablespoons of the toasted walnuts for topping.
Add in the chilled wet mixture and gently fold until a shaggy dough forms.
Rest dough 5 minutes in the bowl.
Pat into a 1-inch thick round (7-8 inches wide) on a floured or non-stick surface.
Cut into 8 wedges with a bench knife (straight down).
Refrigerate on the baking sheet for 30 minutes.
Before baking, brush with walnut milk and sprinkle with granulated sugar. Gently press in the remaining toasted walnuts if desired.
Bake 18-22 minutes at 400°F (204°C), until set and lightly golden at the edges.
Let rest for 10 minutes on the pan, then transfer to a cooling rack and glaze with optional glaze.
Optional glaze: combine powdered sugar, mandarin orange juice, vanilla bean seeds, pinch of salt and vanilla extract.
