I repeated the chocolate experiment using a Trader Joe's UFO Giradelli Chocolate wafer placed on top of a napkin and covered (over the top only) with a piece of aluminum foil. 2.5 minutes of microwaving did nothing to the chocolate.

Then I took a pat of butter and did the exact same thing. After 2.5 minutes it looked a little glossy. After another 2.5 minutes it looked a little bit melted. Another 2.5 minutes and the butter was liquid around the edges.

Then it was time for the "differential measurement" to see if the melting was due to microwaves or heat. (The top of the microwave was warm by that point.) I put a fresh pat of butter of the same size through the same drill only this time the microwave was off. After 7.5 minutes all in one shot on top of the microwave the butter looked somewhat melted. After another 2.5 minutes it was more melted than the first pat of butter. So this test may show a slight difference, but it is not clear.

So I went for a hopefully more exact test: I cut two pieces of butter exactly the same size, by splitting a single slice down the middle. I left one slice on top of a napkin on the counter about 3 ft from the microwave. I put the other slice on top of a napkin and on top of the microwave under a piece of tin foil. Then I set the microwave for 7.5 minutes. 1.5 minutes into the test, I added a piece of chocolate to the napkin on top of the microwave so it got exposed for 6 minutes. After 7.5 minutes of microwaving, the butter was a little bit liquid around the edges, but less so than in the first test. The chocolate was a little bit melted too, leaving fingerprints in the chocolate when I picked it up.

Then I repeated the exact same sequence on top of the now warm microwave with the pat of butter that had been sitting on the counter for 8-9 minutes and a fresh piece of chocolate. The chocolate was placed on the napkin 1.5 minutes into the 7.5 minute test, the same as before. This time the microwave was off. The butter melted almost completely. The chocolate behaved the same as the other piece that was tested for 6 minutes.

Conclusion: The effects of heat overwhelm and confuse any attempt to measure the effects of microwaves on either the chocolate or the butter. Maybe I should try some lard like they use in french fryers. It is solid for long periods of time at room temperautre, thus the test would not be confounded by how long the butter was outside of the refrigerator.