Preheat your oven to 350°F. Grease a 9x13-inch baking dish with butter or nonstick spray and set aside.

Cook the elbow macaroni in a large pot of well-salted boiling water until just shy of al dente, about 1–2 minutes less than the package directions. Drain and return to the pot.

Add the butter to the hot drained pasta and stir until fully melted and the pasta is coated.

Reserve about 1 cup of the combined grated cheeses for topping. Add the remaining sharp cheddar and Colby Jack to the buttered pasta and stir until the cheese begins to melt into the pasta.

In a large bowl or measuring pitcher, whisk together the eggs, evaporated milk, whole milk, dry mustard, garlic powder, salt, and black pepper until smooth and fully combined.

Transfer the cheesy pasta mixture to the prepared baking dish and spread it into an even layer.

Pour the egg and milk custard evenly over the pasta, gently pressing down so the custard seeps through the layers.

Scatter the reserved cup of grated cheese evenly over the top. Dust lightly with paprika.

Bake uncovered at 350°F for 35–40 minutes, until the top is deeply golden and the center is just slightly wobbly when you gently shake the pan. Do not overbake — the custard will continue to set as it cools.

Remove from the oven and let the mac and cheese rest for at least 10 minutes before cutting and serving. This allows the custard to fully set so it holds its shape when sliced.