Preheat the oven to 375 degrees (convection) or 400 degrees (standard). Sift together the flour, baking powder, and salt. Add cubes of butter and use a pastry cutter to completely work the butter into the dry ingredients until there are no large chunks of butter visible. (Mixture should resemble coarse crumbs. Add the cheddar cubes and jalapeño slices and toss to coat.Mix together the cream, milk, egg, and jalapeño juice. Pour over the flour/butter mixture, stirring gently, until the dough all comes together. (It will be slightly dry/crumbly.)Turn out onto a floured surface and use your hands to press into a rough rectangle shape. Sprinkle the top with flour and roll to about 8 inches by 16 inches. Use a serrated knife to cut into 15 squares, then cut each square in half on a diagonal to create 30 small triangles. Transfer to a baking sheet lines with a baking mat or parchment, then bake for 14-17 minutes, or until lightly golden. (Cheese will bubble out a bit. This is fine!)Serve at room temperature with extra jalapeños.