Rewrite A WHERE Clause

I have a WHERE clause that could be an "=" or a "LIKE" depending upon
if the passed variable is populated or not. I would like to know the
best way to write the WHERE clause to make it dynamically switch
between the 2 and make best use of the indexes.

CREATE TABLE myTable(ID INT PRIMARY KEY CLUSTERED, COUNTY VARCHAR(50))
CREATE INDEX IDX_myTable_County ON myTable(COUNTY)

DECLARE @COUNTY VARCHAR(50)
SET @COUNTY = 'SANTA CLARA' -- Could also be SET @COUNTY = NULL

SELECT ID FROM myTable
WHERE COUNTY LIKE (CASE WHEN @COUNTY IS NOT NULL THEN @COUNTY ELSE '%'
END)

This does not seem like best practice to me because I am forced to use
"LIKE" even when @COUNTY is populated with data. Ultimately I'd like:

WHERE (CASE WHEN @COUNTY IS NOT NULL COUNTY = @COUNTY ELSE COUNTY LIKE
'%' END)

but that is incorrect syntax on "=".

Also, I do not want to use a dynamically built statement. Is there a
way around this?

Thanks,
Josh