For the lemon butter:
With the steel blade in place in the work bowl of a food processor, add the sugar and process until superfine. Add the butter cubes and process until well blended, stopping to scrape down the sides of the bowl once or twice. Add the lemon zest and liqueur.
Process until smooth and well blended. Pack into small ramekins and refrigerate, tightly covered with plastic wrap, until ready to serve.
For the biscuits:
Sift together the flour, salt and baking powder in a large bowl. Dump in the butter cubes and work into the flour mixture using your fingertips or a pastry blender, until the mixture resembles coarse meal.
Add about ¾ of a cup of the buttermilk and stir to form a cohesive dough, taking care not to overwork the dough. Add the remaining buttermilk, if needed, to form a soft, shaggy, ever-so-slightly sticky dough.
Turn out onto a lightly floured work surface and knead about 12 times to bring the dough together. Roll out into a rectangle about ¼-inch thick. Using a pastry brush, brush half of the surface of the dough with a portion of the melted butter. Fold the unbuttered side over onto the buttered side, lining up the edges precisely.
Roll the dough out again into a ¼-inch-thick rectangle and repeat the buttering. Repeat 4 more times for a total of 6 turns. Wrap the dough in plastic wrap and refrigerate for 1 hour.
Heat the oven to 425° and line a baking sheet with parchment paper. Roll the refrigerated dough out to ½-inch thickness. Using a sharp 2-inch biscuit cutter and a firm downward cut (no twisting), cut the dough into rounds.
Place the biscuits on the prepared baking sheet and bake for about 10 minutes, or until golden brown and puffy. Serve hot with ramekins of softened Paula’s Texas Lemon butter.