How does the beer look, smell and taste? That determines style, not the recipe.
Sit down with the BJCP guidelines in front of you and taste your beer critically. If it clearly fits into the American Amber (10B) category, then enter it as that. If not, enter it as a specialty beer (23A). If you're still uncertain as to which category you should enter it in, get an experienced beer judge to taste it and give feedback.
If you enter it as a specialty beer, and it's basically a cream ale except for the darker malt character, then declare the base style as "Cream Ale (6A)". For additional comments write something like, "Amber cream ale - similar to a Cream Ale (6A), but with added chocolate malt to impart amber color and hints of dark grain character." If you're certain that the judges will be familiar with the beer reference, and are trying to duplicate a commercial beer, you could also write something like "Muskoka Cream Ale clone."
If the base beer isn't really that much like a cream ale or an American amber, don't specify a base style, just write something vague like "Canadian Amber Beer" which complies with the competition rules. If you do that though, tell the judges what they should be looking for. For example, if the corn really comes through, mention that you used corn. If the Cascade hops are evident, mention them. Don't mention ingredients which don't come through in aroma and flavor, unless you qualify the ingredient with terms like "subtle" or "hints of . . ." For example, "A balanced amber ale with hints of corn. 2-row American pale malt/10% corn, late hopped with Cascade and fermented with California common yeast."