How do I make the top of a part face the mouse?

try this - although I am not sure if it will work;

Part.CFrame = CFrame.new(Part.Position, mouse.Hit.p) * CFrame.Angles(0, 0, math.rad(90))

edit: I just tested it, it works

here's the code i used..

local Part = Instance.new("Part", workspace) do
	Part.Anchored = true
	Part.FrontSurface = Enum.SurfaceType.Motor
end
local mouse = game.Players.LocalPlayer:GetMouse()

mouse.Move:Connect(function()
    Part.CFrame = CFrame.new(Part.Position, mouse.Hit.p) * CFrame.Angles(0, 0, math.rad(90))
end)