Step 1

Adjust oven rack to lower-middle position and heat oven to 450 degrees. Combine garlic powder and water in medium bowl. Add 4 tablespoons butter, salt, and cayenne to bowl; set aside.

Step 2

Place remaining 4 tablespoons butter in small bowl and microwave, covered, until melted, about 30 seconds. Stir in garlic and continue to microwave, covered, until mixture is bubbling around edges, about 1 minute, stirring halfway through microwaving. Transfer melted butter mixture to bowl with garlic powder–butter mixture and whisk until homogeneous loose paste forms. (If mixture melts, set aside and let solidify before using.)

Step 3

Spread cut sides of bread evenly with butter mixture. Transfer bread, cut sides up, to rimmed baking sheet. Bake until butter mixture has melted and seeped into bread, 3 to 4 minutes. Remove sheet from oven. Flip bread cut sides down, place second rimmed baking sheet on top, and gently press. Return sheet to oven, leaving second sheet on top of bread, and continue to bake until cut sides are golden brown and crisp, 4 to 12 minutes longer, rotating sheet halfway through baking. Transfer bread to cutting board. Using serrated knife, cut each half into 8 slices. Serve immediately.