Using a paper towel, grease the bottom of the Instant Pot with 1 tablespoon butter.
Add milk, cream, cream cheese, garlic clove, 1 tablespoon butter, mustard, salt, pepper and cayenne to a blender, and blend until smooth. Add milk-cream cheese mixture and macaroni to the Instant Pot, stirring to combine. Cover and cook on high pressure for 7 minutes.
While the macaroni is cooking, prepare the breadcrumbs. Warm butter in a large skillet over medium heat. Add the garlic and cook just until fragrant (about 30 seconds). Add the breadcrumbs to the skillet along with a pinch of kosher salt and a few turns of black pepper, tossing to coat the crumbs in the butter. Cook until the breadcrumbs turn golden brown (5 to 7 minutes). Take the skillet off the heat, stir in the lemon zest and season to taste with salt and pepper.
Release the pressure manually, then stir in grated cheddar cheese. At this point, check to see if the macaroni is al dente. If it’s still a little too firm, cover and keep the pot on warm for 5 minutes. Stir again before serving.
To serve, divide macaroni and cheese between 4 bowls and top with garlicky breadcrumbs. Enjoy immediately!