Create a stored procedure that can update any column

Here's a simple stored procedure:

CREATE PROCEDURE [foo]
(
	@Id INT,
	@Value VARCHAR(MAX)
)

AS

UPDATE
	[dbo].[Element]
SET
	[Element].[ColumnName] = @Value
WHERE
	[Element].[Id] = @Id

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It works just fine for its purpose. However, what do I do if I want ColumnName to be a passed variable? Something that would work more like this:

CREATE PROCEDURE [foo]
(
	@Id INT,
	@Key VARCHAR(MAX),
	@Value VARCHAR(MAX)
)

AS

UPDATE
	[dbo].[Element]
SET
	[Element].[@Key] = @Value
WHERE
	[Element].[Id] = @Id

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How do I accomplish that task?