Combine all dry ingredients together in a bowl. Add butter pieces and with a mixer (can use either a hand mixer or stand mixer) on medium speed combine the mixture until it reaches the texture of coarse cornmeal. Add the sour cream and 1 tbsp (15 mL) water, and mix again on low speed until the mixture comes together.
Transfer the dough onto a large piece of plastic wrap placed on a work surface. Using the plastic wrap, pull the dough tightly together into a flat wide disc. Chill the plastic wrapped disc of dough in the fridge for at least 1 hour or overnight.
Preheat oven to 400°F (200°C). Roll out the dough thinly between two large pieces of plastic wrap on a clean work surface. Dough should be a scant 1/8” thick or thinner. Using a cookie cutter -- either a square 2" x 2" (5 cm x 5 cm) or oval 2" x 2.5" (5 cm x 6 cm) -- cut dough into shapes and place on a parchment-lined baking tray. Prick each cracker with a fork a few times, brush with the milk and add a small sprig of thyme.
Bake the crackers for 10 to 15 min or until golden brown. Transfer to a cooling rack and let cool completely before storing in an airtight container.
Crackers can be left at room temperature for two to three days or can be frozen for up to two weeks. If softened, they can be reheated quickly in preheated 350°F (180°C) oven for 5 min before serving.