Start by preheating your oven to 425°F. Line a large baking sheet with parchment paper – this prevents the cheese from sticking and makes cleanup way easier.
Grab your jalapeños first. Cut them in half, scrape out the seeds and white membranes, and dice them really finely – you want little pops of heat, not big chunks.
In a large bowl, whisk together your flour, baking powder, salt, and baking soda. Make sure to break up any little lumps of baking powder – they can leave bitter spots in your biscuits.
Take your cold butter cubes and cut them into the flour mixture. You can use a pastry cutter and cut the pieces about the size of peas – some a bit bigger, some a bit smaller. This is what gives you those flaky layers. If the butter starts feeling soft, pop the bowl in the fridge for 10 minutes.
Stir in the shredded cheddar and diced jalapeños until they’re evenly distributed through the flour mixture. The cheese should be distinct pieces, not clumped together.
Make a well in the center and pour in your cold buttermilk all at once. Using a fork, stir just until the dough starts coming together.
Turn the dough out onto a lightly floured surface. Gently pat it into a rough rectangle about 1-inch thick. Don’t use a rolling pin – your hands are gentler and won’t overwork the dough.
Fold the dough into thirds like a letter, then pat it out again. Do this twice more – this creates those lovely layers. Finally, pat it out to about 3/4-inch thickness.
Using a sharp-edged 2½-inch biscuit cutter, press straight down to cut the biscuits – don’t twist! Twisting seals the edges and prevents them from rising properly. Place them on your prepared baking sheet with the sides touching – this helps them rise taller.
Brush the tops with melted butter and pop them in the oven for 12-15 minutes. They’re done when they’re golden brown on top and you can see the cheese bubbling a bit around the edges.