Line a large baking sheet with parchment paper and set aside.
In a large bowl, whisk together the flour, baking powder, and salt.
Add the cold, cubed butter and use a pastry cutter to cut the butter into the flour until pea sized clumps form.
Stir in the jalapeños and cheese. Add the buttermilk and use a wooden spoon to fold and mix it into the flour for 1-2 minutes until a rough, clumpy dough forms.
Carefully tip the dough out onto a lightly floured surface. Squish and work the dough into a cohesive dough ball. It will be very dry and crumbly at first, but should come together after a minute or so. Stop as soon as it forms a ball.
Flatten the dough into a ¾ inch (2 cm) thick horizontal slab.* Fold one of the short sides into the center of the slab then fold the other side over the entire thing.
Turn it horizontally and repeat the previous step two more times.
Flatten the dough one more time into a ¾-1 inch (2-2 ½ cm) thick rectangle. Use a 3-inch (7.6 cm)* round biscuit cutter to cut straight down into the dough (don't twist). You can also use a bench scraper to cut it into squares.
Preheat the oven to 425°F (220°C) and place the biscuits onto the prepared baking sheet. While the oven is preheating, place the biscuits in the freezer for 15 minutes.
Brush the tops of the chilled biscuits with 14 g (1 tbsp) of melted butter.
Bake for 18-20 minutes or until the tops are golden brown.