If you plan to add the same font to several buttons I suggest that you go all the way and implement it as a style and subclass button:
public class ButtonPlus extends Button {
    public ButtonPlus(Context context) {
        super(context);
    }
    public ButtonPlus(Context context, AttributeSet attrs) {
        super(context, attrs);
        CustomFontHelper.setCustomFont(this, context, attrs);
    }
    public ButtonPlus(Context context, AttributeSet attrs, int defStyle) {
        super(context, attrs, defStyle);
        CustomFontHelper.setCustomFont(this, context, attrs);
    }
}
This is a helper class to set a font on a TextView (remember, Button is a subclass of TextView) based on the com.my.package:font attribute:
public class CustomFontHelper {
    /**
     * Sets a font on a textview based on the custom com.my.package:font attribute
     * If the custom font attribute isn't found in the attributes nothing happens
     * @param textview
     * @param context
     * @param attrs
     */
    public static void setCustomFont(TextView textview, Context context, AttributeSet attrs) {
        TypedArray a = context.obtainStyledAttributes(attrs, R.styleable.CustomFont);
        String font = a.getString(R.styleable.CustomFont_font);
        setCustomFont(textview, font, context);
        a.recycle();
    }
    /**
     * Sets a font on a textview
     * @param textview
     * @param font
     * @param context
     */
    public static void setCustomFont(TextView textview, String font, Context context) {
        if(font == null) {
            return;
        }
        Typeface tf = FontCache.get(font, context);
        if(tf != null) {
            textview.setTypeface(tf);
        }
    }
}
And here's the FontCache to reduce memory usage on older devices:
public class FontCache {
    private static Hashtable<String, Typeface> fontCache = new Hashtable<String, Typeface>();
    public static Typeface get(String name, Context context) {
        Typeface tf = fontCache.get(name);
        if(tf == null) {
            try {
                tf = Typeface.createFromAsset(context.getAssets(), name);
            }
            catch (Exception e) {
                return null;
            }
            fontCache.put(name, tf);
        }
        return tf;
    }
}
In res/values/attrs.xml we define the custom styleable attribute
<?xml version="1.0" encoding="utf-8"?>
<resources>
    <declare-styleable name="CustomFont">
        <attr name="font" format="string"/>
    </declare-styleable>
</resources>
And finally an example use in a layout:
    <com.my.package.buttons.ButtonPlus
        style="@style/button"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="@string/button_sometext"/>
And in res/values/style.xml
<style name="button" parent="@android:style/Widget.Button">
    <item name="com.my.package:font">fonts/copperplate_gothic_light.TTF</item>
</style>
This may seem like an awful lot of work, but you'll thank me once you have couple of handfuls of buttons and textfields that you want to change font on.