Wash, hull, and roughly dice the strawberries.
Add the strawberries and sugar to a large pot, stir to combine, and bring to a boil over medium-high heat.
Add butter and boil jam for about 30 minutes. As it thickens and the water cooks off, the bubbling will slow down, and the color will darken to a deep red.
Using a thermometer:
217–220°F.
The plate test: Add a small spoonful of jam to a very cold plate. If the jam feels set when it is chilled you're done. If not, boil for another 5-10 mins.