Melt butter in a saucepan over medium heat
Whisk in flour and cook for 1–2 minutes, stirring constantly
Slowly pour in warm milk while whisking
Continue whisking until smooth and slightly thickened, about 3–4 minutes
Add salt, garlic powder, onion powder, smoked paprika, and cayenne
Reduce heat to low
Add shredded cheese in small handfuls, stirring until melted before adding more
Stir in jalapeño juice if using
Adjust consistency with a splash of milk if too thick
Remove from heat once smooth and pourable