Preheat the oven to 180°C. Line a sheet tray with parchment, spread out the green tomato cubes and bake until they dry out slightly and just begin to colour, 10 - 15 minutes.
Remove and set aside to cool. Raise the oven temperature to 220°C. Sift together the flour, baking powder, baking soda, salt, paprika, garlic powder (if using), pepper and cayenne.
Using a food processor, pastry cutter, or your hands, cut the butter into the flour until it is reduced to bread-crumb size bits.
Beat together the buttermilk and egg, and then gently mix this into the flour-butter mixture until just barely combined.
Add the cheddar, pimientos and cooked and cooled green tomatoes, and gently mix until combined. Place a fresh piece of parchment on the empty cooking sheet.
Turn the dough out onto a floured countertop, and press into a rectangle that's about 1 inch high. Fold the dough over on itself, like an envelope. Give it a 90-degree turn, and press out again to a 1-inch high rectangle (this builds in layers, to make it extra flakey.) Repeat the folding and pressing.
Cut into biscuits with a 2- to 3-inch biscuit cutter, and place on the prepared baking sheets. The biscuits will rise significantly, so leave a couple of inches of space between them.
Brush the tops of the biscuits with melted butter, then bake until lightly browned, 15 -20 minutes. They can be eaten warm or cold.