Preheat the oven to 400°F. Line 2 baking sheets with silpat mats or parchment paper. Whisk the flour, baking powder, sugar, baking soda, salt and pepper in large mixing bowl. Using your fingers or a pastry cutter, rub the cold butter into the dry ingredients until coarse meal forms. Add the goat cheese and buttermilk and stir with fork just until the dough comes together.
Pat the dough out on lightly floured surface to 3/4-inch thick. Using a floured 2-inch-diameter round cutter, cut out the biscuits. Gather the scraps of the dough and pat out again, then cut to make additional biscuits. Arrange on the prepared baking sheets and bake for 14 minutes, turning the pans once during baking, for even cooking. Cool the biscuits on a rack.
To make the Dill Cream, combine the ingredients in the food processor and pulse to combine well.
To serve, split the biscuits in half horizontally. Spread the bottom halves with Dill Cream. Top with a slice of smoked salmon and a few watercress leaves. Spoon a dollop of Dill Cream on top, then cap with the biscuit top.